Rename Message#serialize_to_string to encode, add class-level Message.encode

Prefer the shorter term "encoding".

Setup an alias to the old method name (serialize_to_string) so that we
don't break backwards compat.

Also added a class-level encode method to make it simpler to create and
encode a message (a common use-case).
Rename Message#parse_from_string to decode.

Prefer shorter "decode" method for converting from bytes to an inflated
message.

Add class-level method to create a new object of the message type and
decode the bytes. This in practice is the ONLY use-case for decoding
bytes.
Remove a bunch of old test/example code that is no longer relevant or already covered by specs
Fix bug where ruby < 2.0.0 Range#size is not defined
Set PROTOC_VERSION to 2.5.0
Compile with protoc --plugin in spec_helper
Update gemspec, fixes activesupport < 1.9.3 dependency

* Add devin-c as gem author
* Remove java platform setting
* Remove gem extensions setting
* Constrain activesupport to < 4.0
Cleanup spec_helper
Merge pull request #112 from localshred/bj/encode_methods

Rename Message serialize/parse to encode/decode