Fixed bugs in OptExtract; good progress in making repository Python 3 compatible

[ideasrule]

OptExtract:

• Marsh.c now fully Python 3 compatible
• Fixed bug where Marsh_SObtainSpectrum pointed to same function as Marsh_ObtainSpectrum, and similarly with Marsh_SObtainP. Removed Marsh_SSimpleExtract, since it's not implemented.
• Fixed several mistakes in the documentation and comments
• Fixed several compiler warnings by removing dead code
• Better installation instructions

Python files:

• Ran reindent.py (part of Python) so that all files use spaces, not a mixture of spaces and tabs (which is disallowed in Python 3)
• Ran 2to3 to make all print statements compatible with Python 3
• StringIO import now works for Python 3

Other:

• C files in Continuum are now Python 3 compatible

These changes don't make the repository fully Python 3 compatible--the SSEphem Makefile still needs to be manually changed, for example. Also, I haven't tested these changes much, so beware hidden problems.