Add frange functionality.

frange() takes three parameters (l, u, and field=func) and allows for ranged
function filters by adding {!frange l=%d}func() to ?fq=

see http://wiki.apache.org/solr/FunctionQuery
and http://lucene.apache.org/solr/4_7_0/solr-core/org/apache/solr/search/FunctionRangeQParserPlugin.html
Bugfix: passing of multiple parameters with the same name

This was completely broken. Things like passing multiple facet.field or multiple
stats.facet parameters was not working at all. Only the last one took effect
because I was using a dictionary to store the parameters. Changed this to a list
of tuples, which works equally well for the url encoder
Add frange functionality
Merge branch 'feature/function_range'