Improved `mod_rewrite` check

see https://github.com/open-classifieds/openclassifieds2/issues/845#issuecomment-49769990 

added extra tests & refacto
Improved `mod_rewrite` check

see https://github.com/open-classifieds/openclassifieds2/issues/845#issuecomment-49769990

Added extra tests + refacto
[INSTALL] avoid error when unknown DB is specified
Update messages.po
Merge pull request #846 from emanwebdev/master

[INSTALL] Improved `mod_rewrite` check