Enable or disable bloom filter sync.

When dispersy_enable_bloom_filter_sync is False:
- dispersy_take_step will always be called with allow_sync = False
- acceptable_global_time will return 2 ** 63 - 1 instead of its
  previous calculation involving current global time, mean
  neighborhood global time, and dispersy_acceptable_global_time_range.
Merge pull request #89 from boudewijn-tribler/enable-sync

Enable or disable bloom filter sync.