Add baseclass methods for parsing latest and compatible versions, compatible Firefox support

[osrjv]

I'm not sure what plans you had for this, but this is my small attempt at making the compatible option the default and a shared implementation. The need to call _parse_version() manually in implemented abstract methods is not ideal, but that was the easiest way to not break the API.

Changes include:

• abstract methods for resolving latest and compatible driver versions, with the latter having fallback to former
• baseclass method for parsing the version literals and calling relevant functions
• compatible as default downloaded version
• compatible support for Firefox (needs testing on Mac before release)

[osrjv]

Also, @rasjani, are there plans to make a release soon? I'd like to get these into use and it'd be nice not to have to publish the fork anywhere. I can help if there is something vital still missing.

[rasjani]

cherry-picked into master so no "merge" but code is in .. Will be making a release later today

[rasjani]

Also, not mentioning the new "compatible" mode yet in release, you know its there but i want to run some stuff before publicly advertizing the feature ... but its now in pypi under 0.10.0