Create TMPDIR if it doesn't exist.

The first time I ran "./bin/civiprj create", drush failed because TMPDIR
didn't exist.
Ignore autogenerated snapshot directory.

We don't want to store snapshots in the repository because they are
auto-generated.
Fixed problem where errors were being ignored.

It seems like eval $(cmd) doesn't exit if cmd fails and we want to exit if
the command fails, so we'll store the results of $(cmd) in a variable and
that line will exit if cmd fails, and then we'll eval on the next line.
Removed some repeated code.

PRJDIR/build gets used in several places, so we'll just call it BLDDIR.
Fixed crash because DRUPAL_ROOT doesn't exist.

I'll bet this used to be DRUPAL_ROOT and got changed to WEB_ROOT at some time
and these lines were missed.
Some reasonable defaults to reduce configuration.

We want to minimize configuration as much as possible, so we'll just pick
some defaults for the DB and web site settings. You can easily override the
defaults in build/{$SITE_NAME}.sh.

Also added another override mechanism at config/civiprj.config.sh that works
better for variables you might want to override for all your projects like
CIVI_REPO_BASE.

Ignore the config directory.

It only contains stuff specific to each deployment.