Add the standard Rust Apache/MIT license

This copies the same Apache and MIT license files from the rust-lang/rust
repository to the cargo repository. It notable retains the same copyright line
as the rust repository:

    Copyright (c) 2014 The Rust Project Developers

The COPYRIGHT file from the rust repository was not copied over as it looked
like it mainly contained information about third party dependencies, which cargo
does not have yet.

The wording at the end of the rust repository's README.md was also copied over
to cargo's README.md with tweaks to not mention COPYRIGHT and third-party BSD
licenses.

Closes #34