Docs improvements

Mentioning the restriction on sniffing and GETs with no bodies allowed
(Fixes #137)
Check required parameters for empty values.

Fixes #138, thanks robhudson!
Adding the search_exists API
Make sure we always have connections defined
Add changelog entry for the check in ConnectionPool
Ignore build directory
set propagate=False on elasticsearch.trace only if logger not already configured
Adding the upgrade APIs

Thanks to @ashangit for the PR on which this is based.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	.gitignore
	Changelog.rst