Fix test code coverage, regardless of liboath version
*_validate: update function signatures

* Create oath_toolkit.types.OTPPosition to indicate the relative (and in the
  TOTP case, absolute) position of the OTP in the window on success.
* Remove otp_pos from the args for OATH.totp_validate(), as it never worked in
  the first place, and is now the absolute attribute of the return value.
* Both OATH.hotp_validate() and OATH.totp_validate() return OTPPosition tuples.
* Add test vectors from the OATH Toolkit test suite for TOTP, to test these
  changes.
CFFI: ensure that generated OTPs are the correct number of digits
pyoathtool: make compatible with oathtool 2.4.0

* Add Python3.3 support.
* Update oathtool test runner to download & run oathtool tests from oath-toolkit.
* Integrate oathtool test runner into tox/Travis CI.