Add characteristic descriptors discover command

This command list all descriptors from a specific characteristic.
Add write characteristic descriptor command

We are able to write in a specific characteristic descriptor using that
command. For example CCC (Client Characteristic Configuration). So we
can receive notification of that characteristic.
Add read characteristic descriptor command

With that command, we can read value of a specific characteristic
descriptor.
Define a macro for tool version
Fix extra whitespaces in main() argument
Add register notification command

This patch adds reg-notif command. It is used to register a callback
will be called when a notification/indication is received.

Note: This command does not enable notification/indication in remote
device. For that, you must write 0x01 byte in the respective client
characteristic configuration.
Add unregister notification command

This patch adds unreg-notif command. It is used to unregister the
previous callback that was set by reg-notif command. This way, the stack
will ignore any notification/indication received.

Note: This command does not disable notification/indication in remote
device. For that, you must write 0x00 byte in the respective client
characteristic configuration.
Add rssi command

That commands request RSSI from connected device.
Bump version and update ChangeLog