Add Set schematype
use colander.compat.is_nonstr_iter
empty set is a valid cstruct
Make String serialize empty string to empty string instead of null

This makes the following True:
``` String().serialize(node, u'') == u'' ```

Issue #72
Merge pull request #71 from Pylons/feature.Set

Add Set schematype (backported from deform.Set)
fix formatting typo in the docs
Merge pull request #76 from aiurdoo/master

Documentation fixes.
we arent going to release a 3-dot float-fear colander
Merge branch 'master' of github.com:Pylons/colander
Merge branch 'string_type_serialize_empty_string' of git://github.com/tranminhh/colander into tranminhh-string_type_serialize_empty_string
add change note
make tests pass on 3.2
add Py33 trove classifier
prep for 1.0a1
specify
- Add ``colander.ContainsOnly`` and ``colander.url`` validators.
rename acceptable to choices
prep for 1.0a2