BUG: Avoids install errors caused by lack of pandoc binary.

The pyandoc module throws an OSError of:
"No such file or directory",
when the underlying pandoc binary does not exist.

This error has caused confusion for numerous people during pip
installation.

pandoc is only needed on upload to PyPI to convert to ReST.
So, instead of doing a try/except on the `import pandoc` for all cases,
now checking whether or not 'upload' was invoked when calling setup.py,
so that only maintainers have to worry about installation of pandoc.

By only exercising the pandoc logic when