Convert tweet strings into tweet dicts
Remove text tweet handling in feature selectors
Add missing imports to dbclassify

It looks like it really is a good idea to avoid the
'from module import name' syntax from now on.
Convert text to ascii and filter before normalization

Nonascii chars cause decoding issues later on. Filtering should be done
before normalization for efficiency reasons.
Decode before display in dbclassify

We filter this before insertion, but just in case the DB still contains
invalid chars.