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
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