Call ffi.cdef() for header files in predictable order

Always call ffi.cdef() with a consistent ordering of the header files.
This avoids problems where CFFI generates different checksums (used in
.so filenames) at compile time and runtime, and tries to load the wrong
.so file.

Fixes issue #53 (https://github.com/pyca/pynacl/issues/53)
Merge pull request #54 from wbolster/issue-53

Call ffi.cdef() for header files in predictable order

Closes #53.