Finalize python2/3 transition

[kmuehlbauer]

Although travis runs fine with the current setup, there might be additional changes left.

• make version aware, ENH: make wradlib aware of its version #9

[kmuehlbauer]

@jjhelmus, you may dive in here, if you find time.

[jjhelmus]

@kmuehlbauer Are these any Python2/3 issues in particular you are worried about.

[kmuehlbauer]

For now everything runs fine on travis with the different python versions. So this can be closed. Any bugs that will show can be tackled in the future.