Encode local_page to utf-8

Otherwise we can get "UnicodeEncodeError: 'ascii' codec can't encode
characters in position …" errors
Add --clear-cache option

Allow users to clear the cache.
Return boolean in do_we_have_enough_cookies
Remove stale session from cookiejar
Rename do_we_have_valid_cookies

Because function mutates the cookiejar
Remove retry in _get_authentication_cookies

The stale session is removed in validate_cookies, so we don't need the
`retry` anymore
Merge pull request #209 from jonasdt/cache

Be more careful with stale cookie sessions. From Jonas, closes #208.