Adding amount of water to twitter message  #7

Changes to add the last amount of water that was sent by the arduino to
the database in the twitter message.
bugfix to avoid too long twitter messages

there was an error in checking if the assembled message still is short
enough to be published on twitter.
Changes to adress  Issue #28

Check if the last message sent out by the arduino is a message which
asked for water. If so, make the button to water again visible to the
website.
Changes to adress  Issue #28

Ask for the last message that was sent out by the arduino.