Compatibility to Julia v0.4

Requires Compat.jl
In all my upcoming pull requests, there's a "using Compat" included in the file. Probably that's not the way to do it (I guess there should be a require Compat somewhere when loading the package, rather than a using in every file), but it won't work unless I do this on my machine.

All of this is tested to run with Light Table 0.7.1, Juno 0.9.1, Julia 0.9.2 plugins using both Julia versions 0.3.2 and 0.4.0  
I don't think the `@compat` should be needed here