This allows for adding extensions such as gmail.
Removed categories for mainWindow.  Get the main window from
SimpleAuthProvider.  This helps testing.

Depend on Tidbits, so that we can drop the NSData+Base64
implementation here and use the one in Tidbits (they're both
derived from Matt Gallagher's implementation).  In particular,
the Tidbits one uses ARC.

Use NSUUID to create the nonce, instead of CFUUIDCreate.  Use
gtm_stringByEscapingForURLArgument instead of the
CFURLCreateStringByAddingPercentEscapes-based implementation.

This way the RAC header files get put into the build directory before
we start building SimpleAuth, and so we don't have to reference
the RAC source.

This avoids crashing if we get UIWebViewDelegate events just as
everything is tearing down.

aren't keeping the SimpleAuthProvider instance alive forever.

…schemes.

…arked

as nonnull.