WebStyle: fix StringField definition
WebStyle: fix StringField definition

* StringField in webinterface_handler_wsgi_utils is derived from
  the immutable str builtin type. In order to be initialized
  it requires appropriate usage of __new__ special method instead
  of __init__. (closes #451)

inveniosoftware/invenio-base,inveniosoftware/invenio-base,tiborsimko/invenio-base,tiborsimko/invenio-base