Issue55

Hey there;

I think I have a fix for the issue tracked as Issue #55 on your issue tracker:
https://github.com/nodeca/argparse/issues/55

The issue is that negative numbers in scientific notation are not accepted.  The cause was a regular expression not matching scientific notation; I added a test and tweaked the regex.

I made my changes in a branch called issue55.  I'm still not all that familiar with how things are done around here, so let me know if you have any problems with the work.