Add mode setting for emacs

The syntax for this file approximates javascript closely enough to be useful
More emacs variables
Capture branches/tools and branches/release-tools
Handle Boost.Build's Milestone_12 branch
Move Perforce_Jam_2_4_merge_1 to common_branches
Capture /tags/jam/Boost_Jam_3_1_15/
Capture /branches/build/jam-src/boost/