feat(mime): upgrade to mime v0.3

The new mime crate has several benefits:

- Faster formatting
- Easier to use. Most common mime types are now just constants, like
  `mime::TEXT_PLAIN`.
- Proper suffix support.
- Extensible without breaking backwards compatiblity. This means we can
  always add new constants, but before we couldn't add new variants to the
  enums.
- It's now impossible for a `Mime` to contain invalid tokens. Before,
  with the `Ext(String)` variants, it was possible to create an illegal
  mime.

Closes #738

BREAKING CHANGE: Most uses of `mime` will likely break. There is no more
  `mime!` macro, nor a `Mime` constructor, nor `TopLevel` and `SubLevel`
  enums.

  Instead, in most cases, a constant exists that can now be used.

  For less common mime types, they can be created by parsing a string.

Cargo.toml

@@ -26,7 +26,7 @@ futures-cpupool = "0.1"
 httparse = "1.0"
 language-tags = "0.2"
 log = "0.3"
-mime = "0.2"
+mime = "0.3"
 time = "0.1"
 tokio-core = "0.1.6"
 tokio-proto = "0.1"