Add a config for isort (https://github.com/timothycrosley/isort)
Run isort -rc . and make import style consistent across files
Add a note about `isort` in CONTRIBUTING.md
- fix typo in method name
- make comment more explicit
- test the case where only one element is in the list
- ignore 0 element case (should test skip(), but 2.6 lacks it)
With those changes they can be used to parse plugin API versions too.
Test cases were modified accordingly.
Main changes:
```python
>>> version_from_string("1.0")
(1, 0, 0, 'final', 0)
>>> version_from_string("1.0.1")
(1, 0, 1, 'final', 0)
```
version_info was renamed PICARD_VERSION
version_string was renamed PICARD_VERSION_STR
short version string is stored in PICARD_VERSION_STR_SHORT (for display)
Hooks can be created to handle upgrades (ie. options renaming), see config.register_upgrade_hook() and config.run_upgrade_hooks()
