Improve parsing of plugin api versions and error handling

Display plugin version, api plugin versions, and improve logging.
version_from_string() was made a bit more tolerant so it can be used to parse existing plugins api versions and ease comparaison.

Here how a log looks __after__ this patch (i added few buggy plugins to show each case):
![plugins_log](https://cloud.githubusercontent.com/assets/151042/2778344/376c4674-caf1-11e3-91ff-2eceddc60be0.jpg)

For reference, it was looking like this __before__ (with the same plugins):
![plugins_log_before](https://cloud.githubusercontent.com/assets/151042/2778428/035005b4-caf2-11e3-8a71-d870354b4484.jpg)