types: add enumeration DType (closes #6)

- adds new dependency to package enum34 (pipy) python-enum (deb)
- let the IDE do some formatting
Property: remove value specific parameters from methods (Closes #9)

Parameters like uncertainty and unit were removed ...
- from Property.__init__
- and Property.append
DType: minor improvements

- equal enum name and enum value
- inherit from str in order to make it comparable
  DType.int == 'int' -> True
Merge pull request #22 from stoewer/master

Dtype constants and property constructor
Merge branch 'master' of https://github.com/G-Node/python-odml