To-implement algorithms

[johnnychen94]

@zygmuntszpak When you're available, could you give a reference list of algorithms that needs to be implemented in this package?

[zygmuntszpak]

I have almost finished implementing the subpixel edge detection routines (#9) which I think were the major missing feature that we need to achieve feature parity with what is currently in the Images package.

I haven't kept my dev environment up to date, so I see that a bunch of dependencies are not satisfied with the current pull-request.

The only immedate remaining thing is to implement the edge orientation function with a view towards resolving the unfortunate current ambiguities reported here: JuliaImages/Images.jl#867

I reckon I can tackle that too without too much issue.

In terms of algorithms, I'm not actually sure there is anything major missing. I am not aware of any classic algorithms that work substantially better than canny + subpixel refinement.

The next major bit of work will involve refactoring the Images package to use the edge detection capability provided by this package.