Rename Message serialize/parse to encode/decode

We prefer the shorter encode/decode to more verbose serialize_to_string
and parse_from_string.

Added class-level encode/decode methods to make common use-cases nicer:

```ruby
bytes = Resource.encode(:name => "foo") # is equivalent to:
resource = Resource.decode(bytes)
```

Also removed a ton of old cruft in the test and examples directories.