Implement assignment operators

[fiveop]

The patch implements assignment operators for bitflag structs.

If assignment operators become stable (rust-lang/rust#28235), it can be merged.

Currently, anyone using the bitflags macro would need to enable op_assign_traits for his crate

#![feature(op_assign_traits)]

and anyone using the assignment operators would need to enable augmented_assignments for his crate

#![feature(augmented_assignments)]

This includes the tests and doc tests, thus these fail as long as the feature is not stable.

[alexcrichton]

Thanks for the PR! This is a nightly feature, however, and this is a stable crate, so we can't have this turned on by default. I would be ok putting it behind a feature, however.

[fiveop]

Open a new Pull Request rebased on current master: #35.