Added backtrace-in-coroutine support for OS X.
Merge branch 'next' into sam_165
Removed __MACH__ wrapper around rethinkdb_backtrace in thread_stack_pcs.hpp.
Include execinfo.h for linux backtrace.
Added comment about what part of _rethinkdb_thread_stack_pcs was changed for RethinkDB.
Merge branch 'sam_165' of github.com:rethinkdb/rethinkdb into sam_165
Merge branch 'next' into sam_165