Update to Lucene 4.9.0

Closes #28.

(cherry picked from commit e7f582a)
The README says "with limitations" but there are no docs

We move documentation from http://www.elasticsearch.org/guide/en/elasticsearch/reference/master/modules-memcached.html to the README.
We also add some information about a known limitation (key length 250).

Closes #19.

(cherry picked from commit d4eee11)
Update to elasticsearch 1.4.0

Closes #27.

(cherry picked from commit 74d139c)