Correctly decode bytestrings in Python 3

[bintoro]

Fixes and simplifies Unicode routines. Also adds tests which were
notably absent.

---

Before patch

Python 2:

>>> StringType()('\xc3\xa4').encode('utf-8')
'\xc3\xa4'

Python 3:

>>> StringType()(b'\xc3\xa4').encode('utf-8')
Traceback (most recent call last):
  ...
schematics.exceptions.ConversionError: ["Couldn't interpret 'b'\\xc3\\xa4'' as string."]

[bintoro]

This seems like an uncontroversial bugfix, so merging.