added get_css_count method

I just started to use the module but the test script generated by Perl formatter failed due to lack of get_css_count method.

I updated iedoc.xml to latest one from selenium-2.40.0. The difference between the two versions is only the get_css_count part.

I tried to use create_www_selenium.pl but the POD was not qualified (also checked git log comment says so),  so manually appended the method, a test and the POD.