Make version_to_string() and version_from_string() more tolerant

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)
```
Reformat api_versions list, one per line, and note about order

Order is important since we have now a test case to prevent a lower version to be added.
Use version_from_string() and sets intersection to check API vs plugin versions

Logging was improved, showing plugin version and compatible API versions in debug mode.
When a plugin cannot be loaded due to incompatible API version log as a warning (was info).
No need to traceback on VersionError, just display an error

Traceback isn't useful in this case anyway.
Use path returned by imp.find_module() instead of module.__file__

It will display the source file path (.py) instead of the compiled file path (.pyc)
Log number of plugin names found in each directory
Log when a plugin conflicts with a previously loaded one.

Old one was silently replaced, this debug information will help to know
which one was replaced by which one.

Here is an example of log:
```
D: 12:29:36 Looking for plugins in directory '/home/zas/.config/MusicBrainz/Picard/plugins', 1 names found
D: 12:29:36 Module 'addrelease' conflict: unregistering previously loaded u'Add Cluster As Release' version 0.5 from '/home/zas/src/picard_zas/contrib/plugins/addrelease.py'
D: 12:29:36 Loading plugin u'Add Cluster As Release' version 0.5, compatible with API: 1.0
```
Use constants instead of hard-coded string and number.

"picard.plugins." -> _PLUGIN_MODULE_PREFIX

It also fixes a potential issue forgotten in 5c25d56b7 preventive fix.
Improve API versions list test

- 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)
Merge pull request #304 from zas/plugin_api_versions

Improve parsing of plugin api versions and error handling