Use a more generic way to access mappers, to handle the classic style too
Add almost all the same tests but using the classic mapping style
Fix compatibility with Python 3 in one test

Use zip(seq1, seq2) instead of map(None, seq1, seq2) because the latter
isn't supported by Python 3.
Test suite overhaul, merging declarative and classic suites
Remove duplicated test, leftover from the classic and declarative merge
Merge pull request #9 from lelit/master

Implement support for the classical style, fixing issue #8