Fix min/max validation. Closes gh-666. Fixes #648

In 1.10.0, min/max validation was supported for input type="text",
where min/max were interpreted as numbers.  This means min/max
for date would not work: min="2012-02-13" was interpreted as min="Not a Number".

In 1.11.0, min/max were no longer converted to numbers.  This means
min/max for dates worked, but min/max for numbers failed:
"50" < "150" < "1000" does not hold.

For an example, see http://jsbin.com/awokex/3

This commit makes the behaviour of min/max dependent on input type:

 * input type=text (or not type attribute) has numeric min/max, as in 1.10.0
 * input type=date has working min/max for type date;
   on mobile browsers you also get a date picker,
   plus the browser may reject invalid dates before
   javascript gets a chance to complain.
 * input type=number or range get numeric min/max,
   plus numeric keypad or slider on mobile browsers,
   plus browser may reject invalid input before javascript
   gets a chance to complain

Allowing use of min/max with type=number/range/date is important
for mobile browsers, where the numeric keypad or date picker
make the input much easier to use than a generic text input field.
In this situation jquery-validate remains necessary to support
older browsers that do not do input validation based on type
and min/max.

For situations where numeric input should be validated by jquery
without giving the browser a chance to validate the input format,
input type=text in combination with min/max can be used, as in 1.10.0.