Added support for "required", "maxLength" and "readonly" (#32)

JSON Form now converts JSON Schema properties "required" and
"maxLength" into the appropriate attributes on the input field.

For the "required" property, JSON Form also adds a "jsonform-required"
class to the field container's "<div>" and a "jsonform-hasrequired"
class to the "<form>" tag. The former allows one to flag required
fields easily with a CSS property such as:

  .jsonform-required > label:after {
    content: ' *';
    color: red;
  }

The latter allows one to add a legend at the end of the form, e.g.
with a CSS property such as:

  form.jsonform-hasrequired:after {
    content: '* Required field';
    display: block;
    color: red;
    padding-top: 1em;
  }

JSON Form also supports the "readonly" property when it is set in the
definition of an input field in the "form" section of a JSON Form
object (as opposed to in the schema since "readonly" simply does not
exist in JSON Schema).
Renamed "inputSize" param into "fieldHtmlClass"

The property may be used to specify classes that have nothing to do
with the size of the final input field.