WebMessage: adapt web tests to the new framework

• Adapts WebMessage web tests to the new web tests framework. (addresses #798)

• Implements new class called 'InvenioWebTestCase' in order to build a helper library of useful web test functions like:
  • login
  • logout
  • find_element_by_name_with_timeout
  • find_element_by_link_text_with_timeout
  • find_element_by_id_with_timeout
  • find_element_by_xpath_with_timeout
  • find_page_source_with_timeout
  • element_value_test
  • page_source_test
  • choose_selectbox_option_by_label
  • choose_selectbox_option_by_index
  • choose_selectbox_option_by_value
  • fill_textbox
  • handle_popup_dialog

• Adds "explicit waits" (using WebDriverWait as Selenium people recommend: 'http://seleniumhq.org/docs/appendix_migrating_from_rc_to_webdriver.html') and adds a new class called 'InvenioWebTestCaseException' in order to improve the search for elements on the page:
  • InvenioWebTestCaseException is thrown if an element is not found.
  • WebDriverWait waits up to 'timeout' seconds before throwing an 'InvenioWebTestCaseException' or if it finds the element will return it in 0 - timeout seconds.

• It is recommended to run Selenium 2.7.0; higher versions are more unstable and may 'randomly' fail by timeouting from time to time.