Fetch all items from a channel and then get the first 51
Updating jQuery version
To show channel's posts, we need to sort them by
date. This can be accomplished by using an index
on IndexedDB on the date field, but, we also need
to get posts by the channel name: what leaves us
with only one option, that is to use complex indexes,
what isn't supported by all browsers and also not
supported by backbone indexeddb adapter.

So we choose to add a dirty solution (that must be
updated / changed later): we fetch all items from a
channel, then we sort them based on it's updated date,
and, if needed, we use 'limit' to get the  most
recent ones.
Use posts updates date as since parameter to sync calls and fix a lot of order and similar issues with channel's posts
Merge pull request #289 from buddycloud/master

Master