[Feature] Add Querying capability to Keen Client

### Overview

The Keen Gem did not have a facility to perform queries using the Keen client, and since we (at Ifeelgoods) have been using Keen.io for a while and built our own library to handle querying, we decided to help everyone out there by merging our code into the official Keen API Gem.

### Queries Implemented

The methods: *count, count_unique, minimum, maximum, sum, average, select_unique* and *funnel* are supported (though *extraction* methods are not implemented, it would be trivial to extend the keen querying capabilities to handle the extraction responses).

All querying are performed synchronously (synchronous querying is probably the desired behaviour in most use cases).

### Other changes

Preliminary documentation have been written following the YARD format (www.yardoc.org) for methods #count, #count_unique and #minimum.

Rspec tests have also been implemented for #count and #count_unique.

README.md has been updated with a simple example of using the #count query.

Some changes made based on discussion here: https://github.com/keenlabs/keen-gem/pull/4