Print a message when a command receives a signal

Receiving a signal is normally indicative of violent termination, so the
subcommand can't be relied upon to have printed some status information. As a
result, signals now have some extra errors printed to stderr when they fail.

Closes #261. The actual signal is still a bug, but it's an upstream rust bug.
Add a cargo-doc command
Correctly discover exe suffix through rustc

There was a bug when this was written in rustc that prevented this, but it has
since been fixed.
auto merge of #276 : alexcrichton/cargo/fix-exe, r=carllerche

There was a bug when this was written in rustc that prevented this, but it has
since been fixed.
auto merge of #265 : alexcrichton/cargo/issue-261, r=wycats

Receiving a signal is normally indicative of violent termination, so the
subcommand can't be relied upon to have printed some status information. As a
result, signals now have some extra errors printed to stderr when they fail.

Closes #261. The actual signal is still a bug, but it's an upstream rust bug.
auto merge of #254 : alexcrichton/cargo/cargo-doc, r=wycats

This is blocked until https://github.com/rust-lang/rust/pull/15939 lands, but in the meantime I figured I could get some eyes on this to make sure I'm sane.

Closes #130
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