Keep only v2.3 protocols in master branch

Because STEPPER, ONEWIRE, SHIFT and SCHEDULER are not supported in v2.3 (and then not supported by StandardFirmata), I suggest to delete them from __master__ branch.

For more advanced versions, I suggest : 
 * to create specific branches. Example : 
   * v2.4
   * v2.5
   * v2.6 - Proposals (for proposals only)
 * once the version is supported by main Examples (StandardFirmata, ConfigurableFirmata, etc...), it is merged in master branch.
 * pull request are send to the latest branch, in "Proposal" state

What do you think?

Note : I also add some updates to README file, especially a reference to firmata/arduino.