Make rustc installable with `make install`

[pcwalton]

This depends on issue #528. make install should do the right thing.

[brson]

Still need to make the docs install to the right place, create a man page, and verify it's sane on windows. Also should make sure 'make dist' still works (probably doesn't).

[brson]

make uninstall would also be nice

[brson]

I'm going to call this done and file an additional bug for updating the windows installer.