Installer reloaded

Installer has been totally rewritten in a more flexible and understandable way. form.js code will be reused for the future global form generator that will be included in WityCMS.

Known bugs: (and TODO list)
* [x] Database credentials validation glitches
* [x] Passwords equality check is weird
* [x] Table prefix not checked correctly
* [x] Default admin app list selector has some weird options (. .. or index.html)
* [x] "requires" attribute has a weird behavior on "dbname" if Credentials is not yet validated

Fixes #79 
Fixes #58