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 I'm sorry but a linter that takes 12 seconds to even start is not acceptable for Flycheck.  We'll wait for Julia 0.4 and see if the speed improves to an acceptable level.  Is there an ETA?