Order and Limit queries

I've changed Virsandra::Query class, to make it cleaner and now it is used as superclass for all other query classes. Also limit and order queries added, but not yet used in ModelQuery class. SelectQuery now works a little bit different than before, because now it is possible to chain together more than one where clause and they will be joined together with " AND ", but if also I made old functionality work by adding "reset" method to select query.
I fixed travis config based on this opened issue https://github.com/travis-ci/travis-ci/issues/1053