API - allow fetching of content by health domain or other tag

[alexlittle]

So - for example could get all the resources in a specific language or geography. Currently need to get all the tags, then get request the resources for the specific tag id number.

E.g. to be able to use: http://health-orb.org/api/v1/tags/ethiopia instead of http://health-orb.org/api/v1/102