avoid C dependencies

[rogpeppe]

pymacaroons depends on libnacl, but there's an alternative implementation
of the NaCL crypto that doesn't require C shared library installation (which
can be operationally problematic).

See https://github.com/jfindlay/pure_pynacl.

[ecordell]

I love the idea of this (as it was one of the main motivators behind writing pymacaroons in the first place), but I'm a little concerned with taking a dependency on single-author crypto code that's not actively maintained and afaict hasn't been reviewed by anyone else. libnacl binds to libsodium which is frequently patched and has many eyes and many users.

Another option might be pynacl, which also binds to libsodium but the wheel on pypi includes pre-built libsodium libs. (matrix forked pymacaroons to switch to that).

A third option which would require a little more effort is packaging up pymacaroons with several swappable options for nacl, so you can pip install pymacaroons[pure_pynacl] if you're comfortable with that implementation.

[rogpeppe]

Using pynacl seems like a decent interim solution - that would mean we wouldn't need special apt-get stanzas in various places (particularly as libsodium-dev isn't available directly as under Ubuntu 14.04).

[ecordell]

Pynacl added in #45