A mix task for installing Elks python dependecies.

The various python deps can now be installed to priv/python_deps by
running `mix deps.python`.

Installing them to the priv/ folder should allow them to be packaged
into any releases, simplifying deployment.

PyOpenSSL will still need to be compiled by hand, as it has binary
dependencies that aren't suitable for packaging in this way.
Elk.Config now takes atoms instead of strings.

This fits in with elixir a bit better, and keeps the fact that the
config is in environment variables (and their ELK_ prefixed names)
hidden away as an implementation detail.
Private key file is now a config option.
Example app & a bit of user documentation

Definitely need more documentation, but it's a start.