Viewable timer doesn't stop when element goes out of view

[serbel]

Timer doesn't stop when measured element goes out of view, so MRC is counted when element is not visible.

1. Screencast of the problem on video (example from the IAB repository): https://app.box.com/s/l4bbn5svfyo2byu2o1nswtq3dq7tpdoh
   Preview (example from the IAB repository): http://clients.weborama.nl/files/campaigns2/7533472420/159726/index.html

2. Screencast of the problem on the banner (the last test also shows that the visibility timer constantly stops): https://app.box.com/s/u897trberzhm3753l5n7funmpuyct1qv
   Preview (banner 300x250): http://clients.weborama.nl/files/campaigns2/6383134786/159606/RU_avito.ru_article_201502_index.html

[jdreetz]

The InViewTimer was being re-instantiated without checking that the previous timer had been deleted. This PR fixes the issue - #15