Version checking on startup

[joshrsmith]

@cmarqu Recommended in chat that the Python version be checked on startup and a helpful message be emitted if the version is not supported.

e.g. Check for Major == 2 and Minor ==7 or Major == 3

[cmarqu]

Sorry for not checking this earlier, but using Python 2.6.1.1 gives me now:

Traceback (most recent call last):
  File "examples/vhdl/user_guide/run.py", line 8, in <module>
    from vunit import VUnit
  File "/home/marquardt/tools/vunit/vunit/__init__.py", line 12, in <module>
    import vunit.version_check
  File "/home/marquardt/tools/vunit/vunit/version_check.py", line 24, in <module>
    if not version_is_ok():
  File "/home/marquardt/tools/vunit/vunit/version_check.py", line 19, in version_is_ok
    version = (sys.version_info.major,
AttributeError: 'tuple' object has no attribute 'major'