Remove redundant "extern crate std"

This causes a compiler error on recent Rust versions.
Configure Git to ignore Cargo.lock

The lock file should not be checked into the repository for libraries.
Otherwise the locked versions therein could complicate the build for any
applications using the library.
Fix Cargo warning
Merge pull request #53 from hannobraun/master

Fix compiler error + smaller fixes
Minor fixes to Cargo build.