Added 'doc/' to .gitignore

When using rust-sdl2 git submodule it gets marked dirty when generating
docs in piston-workspace. This prevents new commits.

Ignoring the ‘doc/‘ folder fixes this problem.
Merge pull request #124 from bvssvni/master

Added 'doc/' to .gitignore
Update to latest Rust

renamed container to collection
Merge pull request #125 from leonkunert/master

Update to latest Rust
Merge remote-tracking branch 'upstream/master'