implement utf-8 validation of string fields

only on ruby 1.9+, since it uses the new Encoding features
don't include unset fields in Message#inspect output

fixes #23
fix typo in docs
bump version for 1.2.3.beta2 release
README updates

Reorganize a bit to hopefully make some things clearer.

Added a section describing how to declare and use Message subclasses
without using the .proto compiler at all, using the same DSL that
ruby-protoc outputs internally.
make c ext optional on jruby
Merge remote-tracking branch 'mozy/master'

Conflicts:
	README.md
	VERSION