add isDate validator

I'm using express-form and I found that this is a validator I need. Adding support is easy since it's already a part of node-validator.

I added tests as well, but I can't verify that they work since something else is, as far as I can tell, breaking the test runner.

I get the following (using 'nodeunit' is just a guess, I'm new to node and have no idea what to run tests with):

    spang@sencha:~/src/express-form> nodeunit test/form.test.js

    form.test.js
    ✖ form : isValid

    AssertionError: 'Missing expected exception..' undefined undefined
        at Function._throws (assert.js:301:5)
        at Function.throws (assert.js:318:11)
        at Object.<anonymous> (/home/spang/src/express-form/test/form.test.js:17:20)
        at Object.<anonymous> (/usr/local/lib/nodejs/nodeunit/lib/core.js:233:16)
        at /usr/local/lib/nodejs/nodeunit/lib/core.js:233:16
        at Object.runTest (/usr/local/lib/nodejs/nodeunit/lib/core.js:69:9)
        at /usr/local/lib/nodejs/nodeunit/lib/core.js:115:25
        at /usr/local/lib/nodejs/nodeunit/deps/async.js:508:13
        at /usr/local/lib/nodejs/nodeunit/deps/async.js:118:13
        at /usr/local/lib/nodejs/nodeunit/deps/async.js:134:9


    FAILURES: Undone tests (or their setups/teardowns): 
    - form : getErrors