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
Fix tests for 2.6 support
Add tox configuration file
Add .travis.yml to run tests with tox
Bump up version for release
Drop support for Python 2.5
Try to fix failing test in 3.2
Add Travis badge to README.md