Allow multiple clients

[binaryseed]

I've adapted the Client to be able to start multiple GenServers. I didn't want to change the behavior of any of the other start_link functions, so there's a new function signature...

{:ok, pid} = SomeClient.start_link(thrift_client: thrift_client)

Also included is a test of this behavior, and fixes to a few Elixir 1.3 compiler warnings.

Thanks!

[coveralls]

Coverage decreased (-0.05%) to 96.694% when pulling 2d317e9 on binaryseed:allow-multiple-clients into f4a4654 on pinterest:master.

[coveralls]

Coverage decreased (-0.07%) to 96.68% when pulling c6ec30d on binaryseed:allow-multiple-clients into f4a4654 on pinterest:master.