Thrift: Support for Default Headers and SSL

[nkvoll]

The rationale behind this is pretty much the same as in #1:

Many people use Elasticsearch over unsecured networks that requires encryption and authentication.

Added support for using a SSL-based Thrift socket. Since thrift has no support for authentication in it self, I added the ability to set some default headers on the RestRequests, which is on par with HTTP-like interface of the Elasticsearch Thrift services. This enables, but leaves authentication as an implementation detail in the Thrift server-side, as it likely should be.

I tried to put some consideration into the argument names, but I'm not 100% sure whether they should be prefixed with thrift_ or not, so I left that in.

[coveralls]

Coverage decreased (-0.52%) when pulling 4a8d7bd on nkvoll:master into d7814d0 on elasticsearch:master.

[nkvoll]

And yeah, regarding the coverage, getting integration tests for this might be difficult, and I couldn't find any other unit tests for the surrounding code. Maybe some of these things be tested using mocks where integration testing is difficult, I'm not sure.

[honzakral]

I implemented the SSL support, not convinced we need the custom headers, if I see enough need I will merge it, otherwise I will refer people to subclassing the ThriftConnection class and providing their own logic there.

Thanks for the patch!