Add support for fields parameter in get operation

Update the getDocument method to return the `fields` field instead of `_source` if available.
Allows to get a document from ES and specify the fields you want (See fields section at http://www.elasticsearch.org/guide/reference/api/get/)
Merge remote-tracking branch 'upstream/master' into patch-1
Add testGetDocumentWithFieldsSelection test case
Merge pull request #411 from loris/patch-1

Add support for fields parameter in get operation
Add optimize to status test to make sure all indices are added
Support for "proxy" param for http connections

If proxy passed (empty or actual proxy string), it will
override proxy configuration from environment.
Merge pull request #430 from Topface/http-proxy

Support for "proxy" param for http connections
Implement \ArrayAccess on Elastica\ResultSet

To ease customisation and access to the Results,
we can now get the results manualy.

Not sure about offsetUnset, maybe it should just throw an Exception
Add documentation and tests
Support isset() on results
Release v0.90.2.0
Update to elasticsearch 0.90.3
Nested filter supports now the setFilter method.

When using elastica to perform a search using only filters there was no method
to set an AbstractFilter to the nested Filter so I've added the setFilter method.
Add InvalidException when trying to mess the Result object

refs #445
Merge branch 'master' into array-access-results

Conflicts:
	changes.txt
Merge pull request #445 from damienalexandre/array-access-results

Implement \ArrayAccess on Elastica\ResultSet
When there is no valid connection left, don't retry the request but throw the real error.
Added assertion for __get() and __isset() methods.
Merge branch 'master' of github.com:ruflin/Elastica into patch-1
Support isset() calls on Result objects