Various cleanup + dead code removal (#3)

work around a mysterious chromedriver bug

fix line lengths and import sorting

format code with black

port to python 3

correctly handle both the case when there's a continue button and when there isn't

Freeze requirements

add a couple more random sleeps, and remove the no-longer-present Continue button

update AmzChromeDriver with latest UI changes

add chromedriver to requirements

use new CSS selector for sign-in button

add support for emailing PDF receipts and the Selenium ChromeDriver (replaces mechanize, which does not seem to be able to login to Amazon anymore as of May 2016)

update to BeautifulSoup4 and use it rather than regex for most HTML parsing

support logging in again after failure, and force re-fetch of potentially bad cached data

also credit mechanize in README