made installation instructions more modular for different operating systems
ShellError now detects when an executable is not installed and gives a helpful error message pointing to the documentation
added sensible fallback to pdfminer (a python package) for pdf_parser
added note about installing python header files for osx
moved the installation instructions into standalone url to make it easier to find them
Merge pull request #21 from deanmalmgren/more-robust-operating-system

pdftotext isn't included on non-linux OSes