updating TODO list.
Merge remote-tracking branch 'chris/master'
Merge branch 'master' of github.com:ashima/pdf-table-extract
adding outfilename argument to save colored cells properly
changing install_requires to remove the need for requirements.txt
Github does not use a symlink'd README, copying bits
Merge branch 'master' of github.com:ashima/pdf-table-extract
Adding exceptions to subprocess calls.

1. Before use, a subprocess.check_call is used to check if
pdftoppm is exists and is callable. Exceptions are raised for missing executable and non-zero return codes.
2. Exceptions are checked after each subprocess.popen, not that they catch much.
3. Exceptions in the PNM parsing are raised if the number of parameters in the line
is incorrect (usually because the pdf is empty).