Adding JSON Facet support

[M-Anwar]

This PR adds barebones support for Solr's JSON faceting API. This should help address the issue raised in #55.

The main addition in this PR is including a Facets map[string]interface{} `json:"facets"` to the SolrResponse structure. Along with a few helper methods/types in solrHTTP.go

Example usage is documented in the added unit tests, but also documented below for clarity:

query := []func(url.Values){
    solr.Query("*:*"),
    solr.JSONFacet(
	solr.M{
	    "max_salary": "max(salary)",
	    "salary": solr.M{
		"type":  "terms",
		"field": "salary",
	    },
        },
    ),
}

r, err := solrHttp.Select(replicas, query...)

r.Facets will have the following response structure:

Facets: 
map[
    salary:map[
        buckets:[
            map[val:40000 count:20] 
            map[val:60000 count:20] 
            map[val:80000 count:20] 
            map[val:100000 count:20] 
            map[count:20 val:120000]]] 
    count:100 
    max_salary:120000
]

Because this PR adds barebones support for faceting, additional manipulation of the Solr Facet response is left to the user (i.e conversion to a struct using https://github.com/mitchellh/mapstructure etc).

All features of the Solr JSON Faceting API are supported in this PR (Terms, Query, Range, Heatmap, Metric/Aggregations, Nested, Sorting, Domain (filter, block-join, traversal) )