No it doesn't return an error
Basic `set` with tests

Implementation of a basic `set` which returns the CAS value.
Use `autoexception` for exceptions
`CB_` prefix is not needed
Add `cas` parameter to `set`

It is now possible to supply a CAS as parameter. This way
a key can be refused by Couchbase.

Example:

    cb.set('foo', 'bar', cas=8835713818674332672)
Fix basic.py