Make plugin_root_dir variable be a setting

plugin_root_dir variable might look like a setting in an SetupVAM example, but
actually it was just a shortcut variable which depends on the settings (it must 
be equal to default value in order to work correctly). This commit makes it *be* 
a setting.

Fixes #110
Added bitbucket mirror URL

Ref #106
Allow nested autocommands for vam#SourceMissingPlugins()

Fixes #115
Merge branch 'master' into extended-autoload
Some minor fixes, started making it actually work