Simply (re)naming some variables.

Define api_url, rename riet_json -> riet_issue_json,
in preparation to extending the patch fetching code.
Allow to download "too large to download" patchsets.

Avoid the 404 error on patchsets that are "too large".
Since patch download links are available for individual
files, we donwload them one by one and concatenate them
to obtain the complete patch.

Workaround for Rietveld issue 196:
http://code.google.com/p/rietveld/issues/detail?id=196
Convert integer to str.
Merge pull request #14 from jrioux/rietveld

Rietveld's "too large to download" patches.