Explicit Python Version Support

Used supported versions from http://initd.org/psycopg/docs/install.html
Added explicit Python 2 and Python 3 classifiers

Not sure, but I suspect the "python3" page on PyPy in only based
on the Language :: Python :: 3 classifier, not the more specific ones
Merge branch 'classifiers' into maint_2_5
Fix multiple misspellings
Dropped a lot of words from the front matter

Replaced with links to the types adaptation table and the extension docs.