Acquire webassets via Flask-Assets.

Flask-Assets is currently at 0.8 and requires webassets==0.8,
while webassets==0.9 is the current release. Coaster doesn't ask
for a specific version and 0.9 is the default candidate,
which is breaking installation of Flask-Assets downstream.
Added coaster.utils.nullint for int or None.
Introduced utils.nulstr
Make timestamp columns available without the mixin class.
Recommend parent first in multi-column unique constraints.
Remove set_alembic_revision as a manage option.

"manage.py migrate [env] stamp" does the same thing.
Fix documentation source.
Separate nullstr and nullunicode functions.
Make LabeledEnum.items() sorted by value.
Release 0.4.0 to PyPI and move up to 0.4.1
Slightly friendlier warning message.
Merge branch 'master' into sync_resources
Made sync_resources more verbose