Changes for correct handling in installer:

'''.htaccess'''
 * rewriting changes so a correct URL is passed
'''wikka.php'''
 * Check for MySQL support in PHP before starting (prevents common problems especially with first-time installs)
 * based on .htaccess changes, base_url is now correctly derived, even when starting with a directory URL without trailing slash, or when given a page name but installer is triggered

Some other changes to wikka.php:
 * Utility functions placed at end of file so program flow is clearer
 * some general cleanup and more rational order of statements