Bump branch revision in README
Correct layer name from meta-gumstix-bsp to meta-gumstix in layer configuration.

From the danny release forward, meta-gumstix will aim to be only a bsp layer
with extra gumstix packages and images found in the meta-gumstix-extras layer.
The request to split meta-gumstix layer into two layers comes from this
mailing list thread:
	http://gumstix.8.n6.nabble.com/YoctoProject-Meta-Gumstix-BSP-has-been-posted-td4965534i20.html
Add Gumstix source-cache as premirror.  The cache is hosted by
Amazon cloudfront so it is pretty quick.
Add Gumstix sstate-cache but keep it commented out.

This is an experimental feature at this point. If it consistently
speeds up builds for users without causing problems, then we
should enable by default.
Detect soc-family.  Correct some trailing whitespace and typos.
Bring DEFAULTTUNE into machine configuration for users who want
to play around with different ABIs.  This is experimental so
it is disabled by default.