Deserialize SOLR response in Backend

Avoids duplicate code when processing non-collection
responses (e.g. term lists).
Enable SOLR facetting if facets are requested
Rename: [get|set]Facetting => [get|set]Facets
Merge branch 'master' of github.com:dmj/vf2-search-subsystem
Provide setter and getter for spellcheck
Build spellcheck query (function still needs to be implemented)
Set spellcheck directory if spellcheck is enabled via Params::isSpellcheckEnabled()
Update docstring for getFacets()
Add Backend::setDictionaries()
.gitignore composer.lock