Add a fetcher class to fetch an OAuth2 login scope token from ClientLogin credentials.

Add a fetcher class to fetch an OAuth2 login scope token from ClientLogin credentials.
Full set of tests.

Note that this is not yet hooked up to anything; I am doing the work in peices.

[Rick Campbell, 2011-11-21 21:51:46]

I note that there are a number of lint errors showing in the code review tool. 
More importantly, however, I believe that what is included here should instead
be folded into src/chrome/common/net/gaia/gaia_auth_{consumer,fetcher}*

Specifically, I'd recommended adding this as a new final step in the sequence of
events already driven by the workflow in that class.  Currently that ends with a
request for a ClientLogin user info object.

[Munjal (Google), 2011-11-21 22:14:57]

Regarding the lint warnings, the ones that say no newline at the end of the file
seem to be either a false alarm of due to Windows eol style. I will make sure it
is not the latter. Fixed others.

Regarding the comment about doing this in gaia_auth_fetcher, that class is
already quite complicated. I thought it is worth adding a simple fetcher
abstraction for OAuth2 flows and then we can embed them wherever we want. If we
want ot embed them in GaiaAuthFetcher that is possible too. But the separate
fetcher abstractions for details fo request sending and response parsing seems
better.