Fix --followers typo

Added `_since_def_user` to differentiate if is the user that added the since arg or if is twint that added it for progressing scraping

It seems that even handling the mitigation in the better way, some tweets are filtered out. Twitter search is not so good. Maybe moving the mitigation somewhere else

Added a mitigation because twitter does not work well with given "since" and "until" dates.

Example: https://twitter.com/search?f=tweets&vertical=default&l=de&lang=en&q=%20metoo%20since%3A2017-12-01%20until%3A2017-12-02
this returns tweets until 2017-11-30 which is not good

Added timedelta to choose the time interval for every request, this speed up the process

Changed script in .travis.yml because of timeout

Fixes #48 and #60

implement issue #53 language selection