add the option --enable-local-rust to pull rust from your environment

add the option --enable-local-rust to pull rust from your environment
rather than the snapshots.

make sure to get all of the files.

update to add nmatsakis' requested feature of pointing to a
different rustc install root.

usage: --enable-local-rust to enable
--local-rust-root="/path/to/rustc/" to change the path, which defaults to
"/usr/local/"

Tested on OS X and Linux, likely broken on windows.