Fix 'eups distrib install' breakage caused by 119236b

Commit 119236b stopped exporting setup() and unsetup() functions, breaking
'eups distrib install'.  We now source setups.sh at the beginning of
eupspkg-generated build script to make sure the EUPS environment is
correctly set up.
Merge pull request #28 from RobertLuptonTheGood/fix-broken-eupspkg-install

Fix 'eups distrib install' breakage caused by 119236b