create an "update" command

[jgonggrijp]

setup.py always does the entire installation: detect the location of RED_SPIDER_ROOT, set the environment variables, and install commands. Those first two steps don't need to be repeated when you just check out updated code from a later commit. An "update" command (not necessarily with that name) that is run from within rsshell could do just rebuilding and reinstallation of source files that have changed, if based on the build file as requested in issue #33.

[lramati]

So basically just call a git pull, and then run setup.py?

[jgonggrijp]

Edit: no git pull, because it depends on what branch a user wants to install from.

Just rebuild changed programs + reinstall changed programs.

Note that setup.py does other things in addition to installing, i.e. detecting RED_SPIDER_ROOT, setting environment variables, and in the future possibly creating a configuration files. An "update" command doesn't need to do any of that.