Python 3 compatibility

[lfurrer]

The proposed changes include:

• Python 3 compatibility through explicit relative import statements (contributed mainly by Tilia Ellendorff)
• a setup script (contributed by Lenz Furrer)
• some extended functionality in the bioc_writer module
• improved code formatting according to PEP 8 (a lot of white-space changes) (contributed mainly by Max Nanis in an earlier pull request)
• minor changes/fixes to the small test set

[dhimmel]

@2mh are you maintaining this repository anymore? Python 3 compatibility would be a really nice feature. Really sad to see all these nice pull requests just sitting here.