Get rid of Treebank::Report::Sentences

We can use a plain Report::Generic here.
No need to require Report::Sentences anymore
Move Report::Generic to Common
Enhance alignment spec (red right now)
Allow abstraction of content attr in Reportable

Means something like name='ATR'.
Fix Alignment#report spec
Add base classes for Alignment reports
Change of mind: Revert Reportable#xml_attributes
Fix Alignment#report spec again
Alignment#report green!