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
Add tests for format option
Wrap line (whitespace change only)
Update exception docstrings

`observe` is not implemented yet, the documentation for the
`KeyExistsError` was missing.
Improve `get`

Better error handling and tests for `get`.
Catch exceptions in `set` callback
Move encoding error handling into `_encode_value`
Add `key` to exception
Display error code only if it's applicable
Set `get` callback on init
Make build process clearer
Minor doc fixes
Add multi-set

It's not possible to set several key-value pairs with one
operation:

    cb.set({'foo': 'bar', 'baz': 'value'})
Free allocated memory in case of an error
Add basic multi-get