Add support for multiple facet.field

The facet.field parameter can be specified multiple times to indicate
multiple facet fields [1]. This patch provides support for passing an array
instead of a string on the `field` key of the `query.facet` method
options, eg:

    query.q({'*': '*'}).facet({'on': true, 'field': ['title', 'year']});

as well as:

    query.q({'*': '*'}).facet({'on': true, 'field': 'title'});

[1] http://wiki.apache.org/solr/SimpleFacetParameters#facet.field
Merge pull request #61 from amercader/master

NEW: Add support for multiple facet.field values