Explicit backend
Add a default_backend and start updating docs.
Fix doctests.
Use backend as keyword argument everywhere.
Travis now has an up to date pypy
Merge pull request #273 from alex/pypy-is-now-up-to-date

Travis now has an up to date pypy
Document all the parameters including cross references to specific providers where appropriate.
Test that openssl is the default backend.
Raise a correct error when content isn't padded correctly
Documentation!
One more documentation
Better, but still not covered
include the error message
Coverage
Change teh exception
Useful error message
Merge pull request #274 from alex/handle-bad-padding

Raise a correct error when content isn't padded correctly
Fixed branch coverage
Merge pull request #275 from alex/branch-coverage

Fixed branch coverage
Use keyword argument forms everywhere.