Add summaries to type docs

I didn't get around to adding them for the Set-related and exception classes yet, but all other currently documented classes should be covered.
Also I didn't add them for roles for now, because I'm not yet sure what's the best way to phrase it for them... should the summary for Rational be "Rational number" or "represents a number as numerator and denumerator" or "role for rational numbers"?
Show summaries in the type indexes

It works for sub-indexes, but for some reason not yet for the main type/ index.
Merge pull request #29 from smls/master

Add summaries / short description for classes