fixed formatting issues, added support for screenshot.py

- Fixed formatting issues identified by PEP8 checker, 
- Incorporated comments from @khandelwal 
- Modified class constructors to include a test directory to store images generated by screenshot.py
@khandelwal we use driver_wait in a few places like rate checker:
# This label is invisible on page load
   56          # So we wait for the element to become visble before extracting the text
   57:         WebDriverWait(self.driver, self.driver_wait).until(EC.visibility_of_element_located((By.XPATH, RATE_LOCATION)))
   58          element = self.driver.find_element_by_xpath(RATE_LOCATION)
   59          return element.text