added filter method to INPUT class

the filter() function can be chained between the accessor and the value
function to get a filtered value. When no filter allable is given in the
filter() function, stripctl() is used to strip all control chars (ASCII<32)

Examples:

  $INPUT->post->filter()->str('foobar');
  $INPUT->get->filter('myfilter')->int('baz');
fixed typos in docblock comments
Merge pull request #869 from splitbrain/filter

added filter method to INPUT class