Disable bloom filter sync in Preview- and SearchCommunity.

The new property dispersy_enable_bloom_filter_sync is used to:
1. stop the community from sending bloom filters to sync, and
2. have the community accept incoming messages of any global time
   value.
Add search bloom even though allow_sync parameter is False.
Merge remote-tracking branch 'remotes/upstream/devel' into enable-sync
DHT ready check
Unit tests now use LibtorrentManager.is_dht_ready()
Add DHT check to get_metainfo + require minimum of 25 peers + delete DHT
check from unit tests
Merge remote-tracking branch 'upstream/devel' into dht_check

Conflicts:
	Tribler/Test/test_my_channel.py
Merge remote-tracking branch 'remotes/upstream/devel' into enable-sync
Give more specific reason when search fails.
Merge pull request #217 from boudewijn-tribler/enable-sync

Disable bloom filter sync in Preview- and SearchCommunity.
Merge pull request #221 from egbertbouman/dht_check

DHT check
Merge branch 'devel' into pt