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.
@Wilfred Well, I'd still like to delay this PR until Julia 0.4 is released, and then check again, whether its fast enough.  Currently it's simply not suitable for Flycheck.