Added support for a `size` parameter to facets.

Fixes #192.

You can now limit the number of facets returned by passing the `size` parameter:

>>> s.facet('category', size=5).facet_counts()

Signed-off-by: Kevin Stone <kevinastone@gmail.com>
Added rst documentation for the facet size keyword argument.

Signed-off-by: Kevin Stone <kevinastone@gmail.com>
Updated searching documentation to describe size keyword parameter to facet()

Signed-off-by: Kevin Stone <kevinastone@gmail.com>
Added unit test to validate that facet() passes through the size keyword argument.

Signed-off-by: Kevin Stone <kevinastone@gmail.com>
Merge pull request #193 from kevinastone/master

Added support for a `size` parameter to facets.