Implement tests for WebCLEvent
issue #50

    Change webc-proxy to allow user set device type in
    WCLPlatform::getWCLDevice.

    Change callback function in webcl-test-event to
    increment a global var that cab be validated to
    pass ou fail the test.

    Added instanceof test for WebCLEvent objects.

    Reviewed by Ragner Magalhaes

        * webcl/resources/webcl-proxy.js:
        * webcl/tests/webcl-test-event.html:
        * webcl/tests/webcl-test-event-expected.txt: