Add filtering capabilities to collection fetching

[bcipolli]

According to API docs (http://neurovault.org/api-docs),

Returns a json file containing a list of dictionaries with information corresponding to each collection stored in NeuroVault. Results can be filtered by specifying the name, DOI or owner of the collection.

Parameters: name, DOI, owner

example: neurovault.org/api/collections/?DOI=10.1016/j.neurobiolaging.2012.11.002

This filtering should be available in the API.

[bcipolli]

I see that DOI filtering is available via collections_from_dois(dois) (though it's not accessible via the NeuroVault class). Seems easier to accept filters in NeuroVault.get_collections.