The previous attempt to add unicode support tried to make Python 2 and
3 behave in the same way apart from __str__().  This commit makes the
modules, and the tests, provide only __str__() on Python 3 and both
__str__() and __unicode__() on Python 2.

This commit updates the the screen and ANSI modules to support Unicode
under Python 2.x.  Under Python 3.x, it was already supported because
strings are Unicode by default.  Now, on both Python versions:

- The constructors accept a codec name (defaults to 'latin-1') and a
  scheme for handling encoding/decoding errors (defaults to
  'replace').  The codec may be set to None to inhibit
  encoding/decoding.

- Unicode is now used internally for storing the screen contents.

- Methods that accept input characters will, if passed input of type
  'bytes' (or, under Python 2.x, 'str'), use the specified codec to
  decode the input, otherwise treating it as Unicode.

- Methods that return screen contents now return Unicode, with the
  exception of __str__() under Python 2.x, and __bytes__() in all
  versions of Python, which return the screen contents encoded using
  the specified codec.

These changes are designed to work only with Python 2.6, 2.7, and 3.3
and later, specifically versions that provide both b'' and u'' string
literals.

The check in ANSI for characters being printable is also removed, as
this prevents non-ASCII characters being accepted, which is not
compatible with the goal of adding Unicode support.  This addresses
issue #83.