Detect target file encoding in autodetect.py

Closes issue #65.  Infer the target python source
file encoding before attempting to parse it for
imports of interest.

- First, using byte order markers (BOM).  An example of
  this was found in ``setuptools/tests/script-with-bom.py``.
- Secondly, using PEP263 regular expression over the first
  two lines -- only if they begin with a comment (#), decoded
  as utf8, to determine if it has a source file encoding,
  by seeking statements such as ``# coding: latin1``.
- Finally, default to utf8.  If the target file fails to decode,
  or the given codec is not found -- issue a warning and move
  on to the next file.