Remove all subcommand executables

This commit removes all distributed executables except for `cargo`. All
builtin subcommands are implemented through library calls, and the fallback
methods are retained to maintain extensability through new subcommands.

Closes #393
auto merge of #420 : alexcrichton/cargo/issue-393-2, r=brson

This commit removes all distributed executables except for `cargo`. All
builtin subcommands are implemented through library calls, and the fallback
methods are retained to maintain extensability through new subcommands.

Closes #393