[FEATURE] Python 3 support

- switch to absolute imports
- use print function
- replace cPickle
[FEATURE] Python 3 support

- switch from str to bytes in addons
[FEATURE] replace str with bytes in converters

- use now bytes in all custom converter classes
[TEST] switch from str to bytes in tests
[FIX,BUILD] fix build issue in pyOpenMS

fix Cython code that does not use const in a function which will take
arguments of type "const char *". Adding the const qualifier in the .cpp
file fixes this issue.

fixes #745
[TEST] fix collections and consumer
[TEST] fix integration and memory tests