added two methods to support index aliasing
Merge remote-tracking branch 'binarydud/add_aliasing' into index-aliasing

Conflicts:
	pyelasticsearch/tests.py
ElasticSearch: support index aliasing
Rename modify_alias to update_aliases for some symmetry with update_settings().

We may add higher-level, more convenient methods later called add_alias() and remove_alias() which take more stuff as args rather than making you construct a big dict.

Remove an assertion from the tests that we can't think of a purpose for.
Merge alias support. Closes #76 and #39.