[Core, GTKUI] Added method remove_torrents to core.

Removing multiple torrents with remove_torrents will be faster
than calling remove_torrent for each torrent, as the state file
will be written only once, after all the torrents are deleted.
[Tests] Various fixes for unit tests and tox

* Added custom trial reporter for TODO with test example in test_torrentmanager.py
* Set Stats plugin tests as todo
* Disable new_release_check when running unit tests
* Added pytest.mark.slow to test_core.test_test_listen_port
* Get rid of unit test warnings (Caused by bad names in test classes)
* Removed warnings.filterwarnings in test files.
* Added separate tox target for generating test coverage HTML report.