Command-line option to install as Local System

Added the ability to install the service using the Local System account
from the command-line.
This also requires an update to the docs at:
http://topshelf.readthedocs.org/en/latest/overview/commandline.html to
include the new parameter.
Updated docs for new command-line parameter

Added the new parameter for installing as Local System account to the
docs.
Merge pull request #174 from brutaldev/command-line-local-system

Command-line option to install as Local System