Compatibility with other packages

[willtebbutt]

Upon completion of #29 (and #30 for completeness) it should be possible to make existing packages differentiable automatically. This will be achieved by accessing the source code for the package (locally) and applying a transformation to it such that it uses DiffBase rather than Base and make all of the functions useable with Nodes. This should be instantaneous once #29 has been completed, although lack of support for certain bits of Julia's syntax may show up here, thus it may be necessary to extend certain bits of metaprogramming.

A fairly simple existing package should be identified initially. A reasonably challenging (but rather important) package is Distributions.jl - it is large and will likely throw up any edge cases that I have missed, but making it "differentiable" is a big win.