This also fixes the Python version used for the test and to
install the packages.

* Corrected ICV length for AES-GCM and AES-CCM in IPSec layer. Added unit tests to the ipsec campaign.

* Added travis support for pycrypto 2.7a1 combined modes.

* Updated documentation for pycrypto installation.

  - update (part of) the documentation
    - some URLs for Windows need to be updated
  - update classifiers in setup.py
  - remove `from __future__ import with_statement`
  - use `{k: v for ...}` instead of `dict((k, v) for ...)`
  - remove hacks for `gcd()` and `binrepr()` implementations
  - use class decorators
  - remove Python 2.6 Travis-CI tests