Restore RuntimeError if called outside app context

This is a major error and should throw a big nasty error.

Thank you Python 2.6 for picking up on this. This commit also fixes
compatibility with 2.6.
Backwards compatibility with older Elasticsearch

The official client library for Elasticsearch now has *almost* the same
Python API across two major versions (0.4.x to 1.0.x) allowing us to
provide backwards compatibility with Elasticsearch 0.90.x.

Notes added to the README about how to enable this.