Complex filter expressions in API queries

The current version of the API supports only the AND relation between the fields of the query filters. The idea is to provide the possibility to use both AND and OR relations and build complex filtering expressions with them.

In order to support these complex filters:
* a query grammar should be defined, which
** satisfies the current needs
** is extendable to include further demands later
* the complex filter expressions should be convertible into JSON strings
* POST requests have to be used instead of GET, to be able to send the JSON query filter string in the body of the request
* consider the creation of a new REST resource for queries
** to avoid the difficulties during the modification of the current solution
** keep the API as simple and modular as possible
* choose a DB driver for a proof of concept implementation

The etherpad with further details and comments, can be found here:
https://etherpad.openstack.org/p/Ceilometer_extended_API_query_filtering

Wiki: https://wiki.openstack.org/wiki/Ceilometer/ComplexFilterExpressionsInAPIQueries