bohe implementation for compression tests

I have an initial bohe implementation for the compression tests... it's
very preliminary and uses the same gzip compression as the current spdy3.
I'm going to be playing around with the delta compression mechanism as well
and see how much of an impact that has. Initial results are very promising
but I haven't done much debugging yet. Just wanted folks to know that this
work was underway...

https://github.com/jasnell/compression-test/tree/master/compressor/bohe

Some test runs....

./compare_compressors.py -c bohe -c spdy3 -c delta
../http_samples/mnot/amazon.com.har
732 req messages processed
             compressed | ratio min   max   std
req  bohe        26,122 | 0.13  0.04  0.70  0.08
req delta        33,955 | 0.17  0.02  0.71  0.09
req http1       195,386 | 1.00  1.00  1.00  0.00
req spdy3        27,238 | 0.14  0.04  0.71  0.08

732 res messages processed
             compressed | ratio min   max   std
res  bohe        39,628 | 0.25  0.04  0.66  0.07
res delta        44,499 | 0.28  0.02  0.65  0.09
res http1       159,968 | 1.00  1.00  1.00  0.00
res spdy3        41,325 | 0.26  0.04  0.67  0.08


./compare_compressors.py -c bohe -c spdy3 -c delta
../http_samples/mnot/craigslist.org.har
66 req messages processed
             compressed | ratio min   max   std
req  bohe         1,948 | 0.15  0.06  0.73  0.11
req delta         2,036 | 0.16  0.07  0.71  0.11
req http1        12,894 | 1.00  1.00  1.00  0.00
req spdy3         2,016 | 0.16  0.07  0.75  0.11

66 res messages processed
             compressed | ratio min   max   std
res  bohe         1,786 | 0.18  0.07  0.77  0.13
res delta         2,858 | 0.28  0.08  0.69  0.12
res http1        10,147 | 1.00  1.00  1.00  0.00
res spdy3         1,869 | 0.18  0.09  0.78  0.13


./compare_compressors.py -c bohe -c spdy3 -c delta
../http_samples/mnot/flickr.com.har
438 req messages processed
             compressed | ratio min   max   std
req  bohe        11,988 | 0.10  0.02  0.69  0.07
req delta        26,372 | 0.22  0.01  0.71  0.14
req http1       121,854 | 1.00  1.00  1.00  0.00
req spdy3        12,550 | 0.10  0.02  0.71  0.07

438 res messages processed
             compressed | ratio min   max   std
res  bohe        13,073 | 0.09  0.05  0.66  0.06
res delta        25,236 | 0.18  0.02  0.70  0.11
res http1       140,457 | 1.00  1.00  1.00  0.00
res spdy3        14,142 | 0.10  0.05  0.66  0.06


./compare_compressors.py -c bohe -c spdy3 -c delta
../http_samples/mnot/facebook.com.har
234 req messages processed
             compressed | ratio min   max   std
req  bohe         6,091 | 0.15  0.06  0.78  0.07
req delta         7,800 | 0.19  0.02  0.70  0.07
req http1        41,980 | 1.00  1.00  1.00  0.00
req spdy3         6,301 | 0.15  0.06  0.77  0.07

234 res messages processed
             compressed | ratio min   max   std
res  bohe         9,458 | 0.23  0.07  0.68  0.07
res delta        12,045 | 0.30  0.13  0.60  0.08
res http1        40,252 | 1.00  1.00  1.00  0.00
res spdy3         9,788 | 0.24  0.07  0.69  0.07