README.md: expand the overview, and add a brief build/usage synopsis
protobuf-c/protobuf-c.h: update PROTOBUF_C_MINOR
update copyright and license statements throughout

per https://code.google.com/p/protobuf/source/detail?r=50, the license
on google-originated protobuf code was switched from Apache-2.0 to
BSD-3-Clause (so-called "New BSD" license). update any of the google
license statements to use this new license.

per email with dave, drop the third clause on his BSD-3-Clause license,
so this now becomes BSD-2-Clause.

make sure to use consistent indentation and wrapping throughout.
README.md: add a section on contributing patches