Modernization of code, live demo and ajax support

• [BRK] PHP Requirement upgraded to 5.6 (Faryshta)
• [BRK] Class faryshta\assets\ActiveFormDisableSubmitButtonsAsset replaced by
  faryshta\disableSubmitButtons\Asset (Faryshta)
• [Enh] Added jQuery methods disableSubmitButtons() and
  enableSubmitButtons() (Faryshta)
• [Enh] Live Demo with instructions on [README.md] file

[Bugfix] submit button not reenabling after validation

• [Bugfix] Input submit buttons kept the disabled-text after enabling them
• [Bugfix] Buttons were not reenabled if the css class .has-errors was not used.

first stable release

no issue was reported about the functionality so lets call it stable enough