Update documentation.

1. Change overview picture.
2. Delete sample code.
3. Add meetup slides link.
Remove DummyLog.
Merge pull request #194 from EasonLiao/rm_dummylog

Remove DummyLog.
Merge pull request #193 from EasonLiao/doc

Update documentation.
Fsync log directory when file gets changed.

https://issues.apache.org/jira/browse/ZOOKEEPER-2003
Specify booting parameters in constructor.
Merge pull request #189 from EasonLiao/initialize

Move the initialization parameter to constructors
Merge pull request #107 from EasonLiao/fsync_dir

Fsync directory when files are changed
User decides when to take snapshot.
Raise exception if the size of pending req is large.

Before we blocked the send method.
Merge pull request #190 from EasonLiao/snapshot

Let user decides when to take snapshot
New logic for synchronization.

1. Add subdirectory for log and snapshot.
2. Leader will not send COMMIT to the server who hasn't acked the
proposal.
3. Add firstDivergingPoint method to Log interface.
4. Change the logic of synchronization:

if peerZxid == this.lastZxid:
  send empty diff
else if peerZxid > this.lastZxid:
  if they are in same epoch:
    send truncate
  else:
    state transfer
else:
  if snapshot.zxid > peerZxid:
    state transfer
  else:
    does truncate + delta or diff depends on whether they have diverging point
    or not.
Merge pull request #197 from EasonLiao/sep_data_dir

New logic for synchronization.
Make state transfer atomic.

Create a temporary data directory and store intermediate result there,
after the transferring is done, rename this directory to data directory.

The naming of data directory is like : dataN where N is the id of the
directory.
Fixed bug of RollingLog when call sync() on empty log.
Merge pull request #200 from EasonLiao/atomic_transfering

Make state transfer atomic
Associate context with callbacks.

1. Add context object to method send, flush, remove, takeSnapshot.
2. Pass context object back to corresponding callbacks.
3. Add removed callback (corresponds to remove call).
4. Pass all the pending requests context back to user in recovering
callback to notify them the (possible)failed pending requests.
Merge pull request #199 from EasonLiao/context

Associate a context with request