Extra task added

- Added extra 'prune' task. Useful when bower-rails configuration file
is changed and we want to keep bower_components directory clean after
running "bower update" task. Note: available only with bower >= 1.1.2
- Added 'force' task to "dsl:install" namespace
- Common name convention in tasks description
Merge pull request #27 from romanenko/prune

Prune option for 'update' task