Form_Basic: name fix in addField()

If you call ->addField('dropdown','Is admin?'), then it created HTML elements with name='...Is admin?' which is not correct name.
It should be normalized before using it as part of HTML element name.
https://groups.google.com/forum/?fromgroups=#!topic/agile-toolkit-devel/5a979QuH-vA
Update Basic.php
Merge pull request #279 from DarkSide666/patch-4

Form_Basic: name fix in addField()