General improvements to launch scripts

Now project_env.sh will only define $JOB if it is not already defined.
Also, all the application launch scripts change directories to
$JOB/tmp so that the tools folder doesn't get filled with junk (
this has been happening a lot lately Houdini defaults to using $HIP
to pre-pend a lot of things).

Lastly, put an __init__.py file into the asset_manager directory.
The asset_manager folder will eventually be moved under python2.6libs
and with the __init__.py file it will be treated like a module. This
should help organize things. I will get to this when I have enough time
to make sure that moving the directory won't break anything.
Merge pull request #157 from eestrada/master

General improvements to launch scripts