rustup.

This commit switches to the external `rustc-serialize` crate for
decoding, which causes a breaking change. Namely, instead of using
`deriving(Decodable)`, you will now have to use
`deriving(RustcDecodable)`. Moreover, you must now link to the
`rustc-serialize` crate instead of `serialize`. So change this:

    extern crate serialize;

to this:

    extern crate "rustc-serialize" as rustc_serialize;

[breaking-change]
0.6.19
Change regex for usage section

The regex was matching the command name at any point inside the usage
section. Now, it is no longer multiline and it searches for the program
name at the start of every line. Fixes #48.
Merge pull request #67 from tgkokk/usage-regex

Change regex for usage section
Add version tag.
Add version tag.
0.6.20
Merge remote-tracking branch 'upstream/master'