Add stock framers

This adds a set of stock framers, including:

* IdentityFramer
* ChunkFramer
* LineFramer
* LengthEncodedFramer (abstract)
* StructFramer
* StuffingFramer
* COBSFramer

This also renames framer.py to framers.py to avoid relative import
issues.
Fix Unicode characters issue

Various method docstrings were copied from the asyncio documentation.
Some of these docstrings contained non-ASCII characters, resulting in
errors due to an undeclared encoding on the file.  Rather than declare
an encoding, the docstrings are simply adjusted to use ASCII variants.
Add tests on FramerAdaptor

This creates the test suite with unit tests on FramerAdaptor, along with
fixes for bugs discovered while creating the tests.  This also renames
the send() method to send_frame(), to make its intent a bit more clear.