AccessControl.jl

Hi all,

I've started work on [AccessControl.jl](https://github.com/JockLawrie/AccessControl.jl), which is intended to provide authentication and authorization for web apps written in Julia. A few observations below. Thoughts?

Cheers,
Jock
1. You can see what is done and what is intended by looking at [docs/Contents](https://github.com/JockLawrie/AccessControl.jl/blob/master/docs/Contents.md). I'll flesh this out over the next few weeks.
2. The API is deliberately very imperative. Package devs can then wrap the API in whatever style turns out to be idiomatic for web dev in Julia.
3. It's monolithic. It is modular and could be split into separate packages but:
- A lot of packages would result, confusing newcomers...and me :)
- Most of the time, I suspect all of the time, the functionality of the separate packages would be used in concert anyway, so why not bundle them together in the first place?
4. Further to Point 3, I have included functionality for password hashing and secure cookies in AccessControl.jl. This makes SecureSessions.jl redundant. Should I deprecate SecureSessions.jl in favour of AccessControl.jl (when the latter is built out and tested)?
5. Over time I will harden the security implementation, but at this stage I'm seeking feedback on the API, which needn't change as the security improves.