API adapter update for organisation abbreviations
Show an organisation's abbreviation.

Formtastic calls `display_name` method if it's present. Even 
though no form change has occurred here the displayed organisation 
utilises `display_name` automatically and hence the change to the
Create A Need integration test
Update Needs index view to show organisations abbreviations.

Filtering Needs integration test changed accordingly.
Add `abbreviation` method on organisation

Use attr_reader methods instead of instance variables in the `display_name` method
Add assertion for `abbreviation` method

Simplify tests to avoid stubbing HTTP calls.
Rename method on Organisation model to better describe intent
Merge pull request #44 from alphagov/filter-organisations-by-acronym

Filter organisations by acronym