Add a static assertion that SECRETKEYBYTES == PUBLICKEYBYTES
Fix incorrect API use: pk comes before sk
Add a simple nacl test program

It doesn't do anything very exciting, but it did help catch the sk/pk
ordering bug fixed earlier. It might just be useful in future too.
Finally, real encryption!

This change was so simple that it's almost an anti-climax.
Make gcc 4.4.ancient happy by dropping _Static_assert

Apparently gcc only relatively recently grew support for -std=c11 and
_Static_assert. It's difficult to bring myself to care, but it's a wee
bit inconvenient during testing, so out it goes.

This reverts commit 6748b3b5db4dcc6e4e988a3c5fb5d052c1ed2428.
Don't decode hex digit 'a' as 0

It really helps to not do that. Because that's not what it is.
When we encrypt, we should pass in the zero-padding too
Fix the warning in the README

Now that we have actual encryption working, we can't very well say it's
incomplete.