refactor decode(), fix setup.py for automated sdist builds
* split decode() internals into load() and verify_signature()
* pull code out of read() function in setup.py so it doesn't
  fail when using distutils.core.run_setup() to build an archive
* the setup.py change also uses with so file closing is automatic
add tests for load() and verify_signature()
Merge pull request #25 from dystedium/split_decode

refactor decode(), fix setup.py for automated sdist builds