Initial stub at implementing ThreadSafe::Cache.

The implementation is backed by CHM on JRuby.
Fix an autoload on JRuby.
Warn if defaulting to the synchronized TS::Cache implementation.
Add a simple key retrieval benchmark.
Add ThreadSafe::Cache#clear method.
Add ThreadSafe::Cache#each_pair method.
Add test for cache.fetch {return 123}.
Add for each_pair's iterator like behavior.
Add Cache#put_if_absent method.
Add Cache#delete_and_get.
Add Cache#keys, Cache#values, Cache#each_key and Cache#each_value methods.
Add Cache#empty? method.
Remove extraneous argument.
Get rid of delete_and_get.
Merge pull request #1 from thedarkone/cache

A Cache class backed by CHM
Bump to 0.0.4.
No bugs in JRuby afteral [sic], just me being stupid.
More rbx stuff to ignore.
Make Cache#put_if_absent atomic on MRI/YARV.
Add a note about non-reentrant mutexes.