Add methods to get selected values

So that we have access to both the title and value for the frontend.
Also allow non_described values so that the frontend won't announce
that those values are selected.
Add preposition to be used in result sentence

So that the sentence can be dynamically created add prepositions to the
facets.
Add sentence to the top of results block

Create a sentence about the current filters.
Update tests for new markup
Remove double negative
Use described attribute for readability
Call options 'options' not 'values'
Improve readability
Return empty array rather than nil

So that we can always itterate over it.
Merge pull request #17 from alphagov/search-summary

Search summary

https://www.pivotaltracker.com/story/show/68173466