Add a media type class to OkHttp.

This allows us and our users to parse out the charset of
a content type, so we can cleanly create Readers and Strings
for content of any type.

This is similar to Guava's MediaType but with many differences.

* Guava's MediaType parses all parameters independently. We don't.
  We may later want to support this lazily for users like MimeCraft.

* Guava uses a custom Tokenizer; we use regular expressions.

* Guava includes a registry of common media types; we don't.

* Guava supports media queries and building media types; we don't.

Applications that need advanced features should probably use
Guava's MediaType class. My goal here was to get everything we
need in a small amount of code.