Merge pull request #449 from esc24/cfchecks

Added cfchecker to travis-ci
Area weighted regridding.
More work on regridding.
Reworking coord copying.
More rework.
Fixed units conversion
Move to iris.experimental.regrid
Broken bounds wrapping
Use of axis keyword in average. This needs numpy v1.7.
Start of tests.
More testing.
More corner cases for cropping bounds.
More tests.
Support for regriding to scalars.
subsampling test helper
Pep8 tweaks.
Partial review.
Review actions.
Tidy post rebase.
Further work following rebase and review.