ENH: Replace has_key() for dicts with in operator

The method has_key() for dict objects is deprecated and not supported in
Python 3. For improving upward compatibility three uses of has_key() for
dicts not leaving the functions have been replaced with the in operator.
ENH: Replace has_key() for dicts with in operator

The method has_key() for dict objects is deprecated and not supported in
Python 3.x. To improve upward compatibility several uses of has_key() on
dicts in class object attributes have been replaced with an in operator.
ENH: Correct typo in source comment
DOC: Correct typo in documentation
Merge pull request #418 from philippwiesemann/master

ENH: Replace has_key() for dicts with in operator