Feed (rss/atom/rdf) importer

Adapted from http://github.com/mjibson/goread.

Change-Id: Id3b61b155624d8c34aadd77a82c40ea4d44d66c0
importer/twitter: refactor credentials isolation

-store in the importer account the temporary credentials needed
during setup.
-store in the importer account the access token, and the access token
secret, and guard them. They are needed at the end of the setup (to get
the user's info) and all along a run.
-not share the same oauth.Client with the whole package. Create a new
one everytime it's needed in the setup. Create a new one at the beginning
of a run too, and it's ok to reuse it all along, since it's used
read-only.

Change-Id: I4823b35abf8a5170f63ca502026f6c8f98e2c1e7
Merge "importer/twitter: refactor credentials isolation"
twitter: add oauthContext to clean up some function signatures

Change-Id: I301d2ed8ee68aa316349c44bf0d20e4fcaacf99e
Merge "Feed (rss/atom/rdf) importer"
Add more to 0.8 release notes.

Change-Id: Ic84087ab7fa0e35d81c146c1ebf61426873c548b