[fix] Uses shared pointers instead of explicitely shared objects

Shared pointers at least reflect the syntax of a pointer,
so Node and CacheEntry are now migrated to QSharedPointer,
instead of the old and dirty explicitely shared objects.

There is also a quick fix for one test included.
[feature] Implemented downstream replies for Twitter
Fix for page handling

Page handling was partly broken before, since, when
pushing data after or before in a SNMI caused information
loss about pages.

Specifically, if we consider data that have previous
and next data, we store the previous cursors and next
cursors in Node::extraInfo.

However, when newer data got pushed (for example appended),
newer cursors for _both_ next and previous got inserted
in Node::extraInfo, so we lost information (in this example,
the cursors for prepending data).

This caused problems for navigation if a data stream supports
both loading next and previous pages.
[feature] Comments fixup

This commit provides a filter that is able to retrieve
comments from Facebook using offsets. It also provides
threaded comments in the main stream by default.

Also added a small protection, for not loading next / previous
when hasNext or hasPrevious is set to false.
[bugfix] Fix a bug in error handling
Merge pull request #36 from SfietKonstantin/twitter-conversation

Twitter conversation and Facebook comments
Merge pull request #37 from SfietKonstantin/error-fix

[bugfix] Fix a bug in error handling
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