Increase python requirements to python 2.7

Once we have a proper ansible + conda environment from which to run bliss we can use this MR to clean-up all garbage which concerns python 2.6.

Depends on #181 (closed).

• increase the minimum python version to 2.7
• make a strict requirements.txt
• in setup.py
  • python version check
  • add install_requires
• add install_spec command to setup as extension