Add failing test
Do strict type application, if necessary
Apply offset when doing type guessing
Merge branch 'master' into 88-offset-bug

Conflicts:
	dataconverters/commas.py
Apply offset when guessing xls
Require latest messytables
Merge branch 'master' into 88-offset-bug

Conflicts:
	requirements.txt
Merge pull request #88 from okfn/88-offset-bug

Header offset guessing does not work with type guessing