also, convert types to int when generating line from str

also complete classes docstring

* Move logic in generate to class
* Add test
* Move 7500 to global constant
* Make read_started_at not fail when conf is None

    * Move V3BWLine to v3bwfile.py
    * Add methods to V3BWLine similar to V3BwHeader
    * Add methods to V3BWLine to implement most of the logic in the class
    * Move functions related to data lines to v3bwfile.py
    * Add functions to generate types of erros
    * Add tests for the class and conftest to read results from file
    * Fix other tests depending on bw lines

so that it close to the header and we can reuse methods/functions

also:
* fix again number of header lines
* FIXME: tests for bw lines should be independent on headers,
  Otherwise they'll fail everytime we change headers.

also:
* remove latest_bandwidth outside of class, since it should be
  generated from timestamp
* pass all init arguments as string
* require timestamp argument
* fix tests with changes

they do not need to be in 3rd and 4th posstions.

Also:
* add latest_bandwidth header
* add file_created header
* fix bw files header tests
* disable tests that assume timestamp to be the file creation
time
* if we use ISO 8601 with T as separator,
  datetime format string is not needed

On one hand, for weeks now I've been running sbws and it seems to be
correctly doing downloads over tor. I've monitored existing circs and
streams with a script that queries the control port and what I saw made
sense.

On the other hand, I just tried running sbws on my laptop over a VPN
that blocks resolution of a hostname of mine. I got a proxy error with
a suspicious hostname in the message that led me to believe that
destination usability checks were not actually being done over Tor like
they are supposed to be.

The way Requests tells you to use proxies is by specifying them *not* on
the session object, but as an argument[0]. So do that.

[0]: http://docs.python-requests.org/en/master/user/advanced/#proxies

remove unused key tor_control_socket
remove duplicate key started_filepath in scanner section

GH: closes #139