Promote Box to a class and check parameters

A valid box must have equal dimensions in position and size. It also may not
have a negative size.

Fixes issue #23.
Merge branch 'master' into feature/box_class

Conflicts:
	jelly/tests/test_util.py
Fix missing import of nose.tools.raises
Merge pull request #44 from aepsil0n/feature/box_class

Promote Box to a class and check parameters