1-8：8种常用的元素定位方法
1.find_element_by_id(self, id)
2.find_element_by_name(self, name)
3.find_element_by_class_name(self, name)
4.find_element_by_tag_name(self, name)
5.find_element_by_link_text(self, link_text)
6.find_element_by_partial_link_text(self, link_text)
7.find_element_by_xpath(self, xpath)
8.find_element_by_css_selector(self, css_selector)
9-16:以上8种的复数形式
9.find_elements_by_id(self, id)
10.find_elements_by_name(self, name)
11.find_elements_by_class_name(self, name)
12.find_elements_by_tag_name(self, name)
13.find_elements_by_link_text(self, text)
14.find_elements_by_partial_link_text(self, link_text)
15.find_elements_by_xpath(self, xpath)
16.find_elements_by_css_selector(self, css_selector)
17-18:
17.find_element(self, by='id', value=None)
18.find_elements(self, by='id', value=None)
