fix(INFile): address changes to API, add method to fetch file data

INMessage#getAttachments() will fetch files attached to the message from the server, using the
typical motif of these API request methods (supports passing in an array of files to update,
as well as an object of filter keys).

INFile#download() will attempt to download an attachment asynchronously (requires XHR2)

INMessage#attachmentIDs has been removed, and replaced with INMessage#attachmentData, containing
all of the data for each file.
fix(INNamespace): correct url for INNamespace#tags()

Previously was requesting /n/<namespace_id>/threads, should be /n/<namespace_id>/tags
fix(capitalizeString): return value from capitalizeString()

Also adds tests which were previously missing.
chore(namespace): Converting tabs to spaces