Improvements to install script

* automatically remove install/ folder
* go easier though the compatibility test <-> install script pages
* Small fixes which raised php warnings
[fix] undefined vars in install script
Merge pull request #588 from mariroz/dev

fix of bug with default theme constant
oops, restore @tcitworld commit. I'm so tired
Merge branch 'dev' of https://github.com/wallabag/wallabag into dev
Important fixes for search engine (thx @mariroz)

So sorry for the mess... :(
* search only in users' own articles
* sanitized what is searched
* display what is searched
* pagination, sorting available when searching
* use existing function to query db
* bad encoding caracters fixed
* link to JQuery into default theme, no longer in each theme
* some spaces instead of tabs
import without cron
import without cron