Adds example and a testable docstring for assert_diff

[petr-tik]

Runs rustfmt on the lib.rs

Closes #35

[petr-tik]

Johann - this is an awesome library that makes it easier to test the equality of 2 strings! Thanks for your hard work.

I find rust macros v powerful, but am not that proficient with them. I find it useful to have documentation and examples of macros.

I hope this addition helps other users find examples of the convenient macro that you added.