encode: use an on-stack buffer for 2-char escape sequences

When profiling my application, this sped up yajl_string_encode by
3.9%.
encode: don't check specific cases when the current char is large

Most characters don't get escaped.  According to the RFC, the only
character greater than '/' that gets escaped is '/'. Test for this
common case first, and only if the char is a candidate for escaping
check for the other escape cases.

This cuts an additional 29% off the runtime of yajl_string_encode
augment in-tree performance benchmark to assess serialization performance as well as parsing performance.  (provides a means to assess issue #59)
bump yajl version to 2.1.0 on the 2.1.0 branch
Merge branch 'encode-speedup' of https://github.com/bpowers/yajl into 2.1.0
update changelog with @bpowers contributions
Merge branch 'master' into 2.1.0

Conflicts:
	CMakeLists.txt
	ChangeLog
Merge remote-tracking branch 'upstream/2.1.0'