Implement a simple ID range as storage

This is a first step in refactoring the ID assignment logic currently used.
Implement compound ID assignment

Can assign IDs from multiple ID providers using a dispatcher class.
Discard Mesh class in favor of simple function

The Mesh class' simple functionality did not justify its bloat. It also hid
away some details that can used in a more modular way. Therefore it is hereby
replaced by a simple factory function that produces a list of cells instead.
Use new ID assignment for IC writing
Adapt test MD5 hashes to rebase
Add mock to tox configuration
Fix dict methods to use six
Monkey-patch custom assert_is for Python 2.6
Merge pull request #52 from aepsil0n/refactor/id_ranges

Refactoring of ID ranges