Update partitioning-big-data.txt

Add in a note about the default parsing behaviour of load-graph.py when there is no recognised file extension.
Remove argparse from the requirements

unless it isn't available. Argparse is bundled with Python 2.7+.
This simplifies the installation instructions. Tested with cpython 2.6
& 2.7 on Ubuntu 13.10.
Merge pull request #401 from ged-lab/fix/argparse-nodep

Remove argparse from the requirements
Merge pull request #400 from ctSkennerton/patch-1

Update partitioning-big-data.txt
Explicit python2 invocation