sync _loadedEntries and _entryViews

These two lists need to be in sync. This constraint was temporarly
violated on item addition. Meld all the modifications into _insertItem,
so that they are guaranteed to be synchronized.
replace innerHTML with using parserUtils

This requires at least Firefox 14. In-item scripts are disabled, in-item
styles and external embeds are still enabled (may be disabled later).

Fixes #48 by disabling the in-item JS. However, #47 is likely not fixed
due to autoplay in <audio> and <video>. May lead to performance changes.
Affects #9 as relative links are now disabled completely.
no need to remove document.write without scripts
Use grayscale icon, if all feeds are readed
switch tame representations based on elapsed time

The elapsed time is a better measure than the number of clock value
changes.