Added 'all' Method which corresponds to /simple/all

Added 'Method' to get all Documents from a Collection.
This Method corresponds to the /simple/all Method see: https://www.arangodb.org/manuals/current/HttpSimple.html#HttpSimpleAll
Added Tests for CollectionHandler@all

3 new Tests for 'all' Method in CollectionHandler Class.
Tests all Method, all Method with 'skip' Parameter and all Method with 'limit' Parameter.
Added constant for /simple/all request

Constant for ArangeoDb request /_api/simple/all
Merge pull request #143 from Beldur/devel

Added 'all' Method which corresponds to /simple/all