[User Timing] Spec updates

The following updates were made to the User Timing spec, http://dvcs.w3.org/hg/webperf/raw-file/tip/specs/UserTiming/Overview.html:


-          Fixed typos, grammatical issues and links.

-          Added references to DOM Level 3 Core and ECMA-262 specs

-          mark() throws a NOT_SUPPORTED_ERR exception if the name of the mark is the same as an attribute in the PerformanceTiming interface.

-          measure() throws a NOT_FOUND_ERR exception if the name of a mark doesn't exist.

Please review the changes.

Thank you,
Jatinder Mann