Fix #775 js validation when not setting password

changed indentation, actual code change is line 39. Don't validate the
password until someone types in the password field.
install: remove unreliable check for mod_rewrite (issue #772)

Unfortunately this check doesn't appear to be reliable.  The information
was provided by PHP's apache2handler.  I guess one explanation is that a
different handler was being used (generic cgi/fastcgi).

The removed comment may be confusing, so note that mod_rewrite is an
Apache module and not a PHP extension :).
role model: prevent deleting default role

Undesirable consequences of deleting the default role are mentioned in
issue #767.
Fix #758 core module overrides
install: change default DB driver to 'mysql' (see issue #765)

The current version of CodeIgniter's 'mysqli' driver doesn't support the
backup() method.
install: avoid accidentally re-creating installed.txt (issue #761)

The installer pages aren't as paranoid / robust as they could be.
This commit just tweaks one case that bit me.

If you install, then remove installed.txt, then re-visit
install/index.php/complete (which shows a fair amount of information you
might have been interested in) - it causes installed.txt to be re-created.

Move the code that creates installed.txt so it's somewhat less likely to
be triggered.  With this tweak, you'd have to go back a second time to
the "do_install" url (the method that creates the database, among other
things).
Merge remote-tracking branch 'upstream/develop' into develop