Add regular expression for internet media types.

[amoskopp]

Add a regular expression that validates against (current and possible) internet media types. This is not an enum because future media types might exist. The regex might look a bit weird because of JSON escaping rules.

[amoskopp]

Just to add why I think this PR is appropriate: I suspect that without a regex in the schema, people are going to validate media types using something like \w+/\w+ – which is simple, but wrong.

[kevinswiber]

@nmoskopp Thanks!