expose and test crypto_scalarmult

fixes #65
scalarmult: remove unreachable error-handling code

The two scalarmult functions always return 0 (they should really have
been declared as 'void'). So simply assert that they've returned 0
instead of trying to raise an exception in a code path that will never
be exercised or tested.
Merge pull request #67 from warner/scalarmult

expose and test crypto_scalarmult