Add slotting to rust ebuilds.

These slots are added:
(major.minor) rust release
(nightly) nightly version
(git) git master branch

eselect module was added, that allows switching between different rust versions

To make all this work, proper support of --libdir option in rust build
system was implemented (previous version did not work), see
rust-lang/rust#16552
Until upstream accepts it, it will be maintained downstream
Merge pull request #28 from jauhien/add_slotting

Add slotting to rust ebuilds
[app-admin/eselect-rust] version bump: 0.1_pre20140820
[app-admin/eselect-rust] 0.1_pre20140820: add pkg_postinst/pkg_prerm