Feature/doc explain

fixes #500 adds support for the Explain API 

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-explain.html#search-explain

This also fixes an important bug where the queyparameters for SourceInclude, SourceExclude were incorrect. 

This also deprecates `SourceEnabled(string[] )`  for `EnableSource(bool)` while the latter is technically correct according to spec in practices it has no use. The spec marks it as lists but it accepts a bool or list and the bool is the common use case.