Add new interface for handler

Now library user need to use GerritHandler directly to handle event.
It has possibility for user to call unnecessary method.

To avoid this, this patch provides new interface class to use
GerritHandler safely.

Also new connection event type is provided.
Implement Handler to GerritHandler

This patch implements Handler to GerritHandler.

Some methods are deprecated. And GerritHandler will not provide
connection status anymore.
Add overload method for JSONObject parameter
Add important comment

Some methods pass parameters to the constructor of the Work class.
It is referenced by other thread. Thus it is expected that objects
referenced by parameters are not updated.

So this patch adds a caution to method document.
Add new method to check interesting and usable for JSONObject

For new Work class, interesting and usable check for JSONObject is
isolated from original method then put it to new method.
Add new Work class

For handle JSONObject directly, new Work class "JSONEventWork" is added.
Implement addtional method for handler
Integrate post logic to one method

Handler has many similar methods. It means that many similar logics
could be put to implementation class.

This patch integrates post logic to one method to prevent logic
duplication.
Remove unnecessary import

To fix checkstyle error.
Add Japanese translation for GERRIT_CHANGE_COMMIT_MESSAGE help

Change-Id: I239d3d9b5b297141f406c9cc06adc6d6ac3e9d91
Minor updates in the Japanese help file

- Consistently capitalise "Gerrit"

- Add missing translation of `GERRIT_BRANCH` description.

- Add the "only if Gerrit version is 2.8 or higher" caveat on the
  description of `GERRIT_CHANGE_COMMIT_MESSAGE`.

Change-Id: Ie1e97cdb4cb04bc513e4ef9333a6e99c33036fec
Add new interface for handler

Now library user need to use GerritHandler directly to handle event.
It has possibility for user to call unnecessary method.

To avoid this, this patch provides new interface class to use
GerritHandler safely.

Also new connection event type is provided.
Implement Handler to GerritHandler

This patch implements Handler to GerritHandler.

Some methods are deprecated. And GerritHandler will not provide
connection status anymore.
Add overload method for JSONObject parameter
Add important comment

Some methods pass parameters to the constructor of the Work class.
It is referenced by other thread. Thus it is expected that objects
referenced by parameters are not updated.

So this patch adds a caution to method document.
Add new method to check interesting and usable for JSONObject

For new Work class, interesting and usable check for JSONObject is
isolated from original method then put it to new method.
Add new Work class

For handle JSONObject directly, new Work class "JSONEventWork" is added.
Implement addtional method for handler
Integrate post logic to one method

Handler has many similar methods. It means that many similar logics
could be put to implementation class.

This patch integrates post logic to one method to prevent logic
duplication.
Remove unnecessary import

To fix checkstyle error.