Use custom exec() to discover version in setup.py

To prevent possible race conditions (?)
Store more metadata in udiskie/__init__.py
Merge branch 'setup-metadata'
Use UDisks2 by default

The udiskie.uDisks2 backend is mature enough to be used by default now.

Advantages of UDisks2 are:

- the asynchronous UDisks2 API is less prone to race conditions
- it is easier to work with in the context of the recent changes to make
  udiskie itself operate asynchronous
- support for UDisks1 is slowly dropped from some distributions