Adding a Julia lint checker along with tests.

Fixes #502.

This checker can be slow (12 seconds on my old laptop), however this will improve hugely in the next version of Julia. It would be wonderful to see this in flycheck, let me know if you'd like me to change anything.
On Julia 0.3, it takes around 12 seconds to start the linter, as it JIT compiles all the code imported, every time. In 0.4 performance will be much better. https://github.com/tonyhffong/Lint.jl/issues/46