Add a test for existing child tags feature
Allow tags to be sorted in alphabetical order

When tags are requested at /tags.json, we should be able to request them 
in alphabetical order. This saves us from doing an in-memory sort in the 
client (eg the browse pages in Frontend).
Make clearer that titles are being sorted, not IDs

In the test for tag sorting behaviour, use numeric-ish tag IDs, which 
are different from the tag titles, so that the testing behaviour is 
clearer.
Merge pull request #148 from alphagov/sort-child-tags

Allow tags to be sorted in alphabetical order