Feature/add gomemcache

Add optional golang based gomemcache library. 
This library does not require libmemcached, and provides it's own pooling, but does not offer the level of control that gomc does. This library has NOT yet been fully load tested.

@bbangert || @kitcambridge r?
Could the pool be created via `s.client = mc.New(s.Hosts...)`? Looks like the library [manages a free connection pool](https://github.com/bradfitz/gomemcache/blob/master/memcache/memcache.go#L195-207), so that might remove the need for `{get, new}Client()`.