Move from hammer to docopt for option parsing

The hammer library currently has some shortcomings such as the inability to
document individual options. Additionally our handling with hammer of extra
arguments is dodgy at best currently.

This commit moves the repository to BurntSushi's docopt.rs library which seems
to more feature-complete at this time. Additionally, docopt has the great
benefit of a "document once, use everywhere" documentation strategy.

This migration solves two primary issues:

* Comprehensive and useful CLI documentation
* Gracefully handling flavorful combinations of arguments in odd combinations

Closes #218
auto merge of #266 : alexcrichton/cargo/docopt, r=wycats

The hammer library currently has some shortcomings such as the inability to
document individual options. Additionally our handling with hammer of extra
arguments is dodgy at best currently.

This commit moves the repository to BurntSushi's docopt.rs library which seems
to more feature-complete at this time. Additionally, docopt has the great
benefit of a "document once, use everywhere" documentation strategy.

This migration solves two primary issues:

* Comprehensive and useful CLI documentation
* Gracefully handling flavorful combinations of arguments in odd combinations