Added maxcount option to the Multichoice field.

This field allows to set the meximum number of selections
a user can make in a choice-multiple field.
Made Question#extra field max_length larger (128 -> 512).
Bugfix - iftag="" did not work for a question that is in a questionset, in which other questions should be visible.

In that case, even though the question did not fulfill the iftag requirement it was shown.
If the user is using ?show_all=1 GET parameter, no fields are hidden.

Even if requiredif/iftag doesn't allow to show them.