[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
Merge pull request #749 from hroest/feature/Python3

Feature/python3
[FIX] VS2013 compile fixes
[FIX] also add MSNumpress python test to list of tests

- ammend commit 9500476d59d38b5ffe0520d2d7fec8cc52c91d19 (r12099)
Merge pull request #785 from timosachsenberg/fix/VS2013CompileFix

[FIX] VS2013 compile fixes
Merge branch 'develop' of https://github.com/OpenMS/OpenMS into fix-idpep-mascot
[FIX] replaced 'DoubleReal' by 'double' in accordance with commit 4d40981a248cf4757f2e68668361f8eb68c40c99