js: created TestExtension and smaller fixes

• Extensions:
  • Moved testapplication to Demoextension
  • Created BrowserTestExtension. Includes mocha and chai. For running tests that cannot be run outside a WebExtension
    
    Both Extensions can be found zipped in build/extensions after running build_extensions.sh

• Code changes:
  • src/Config: Place for the configuration
  • small fixes raised during testing in Keyring.js, Message.js,
  • src/gpgmejs_openpgpjs.js don't offer direct GpgME object to the outside, as it only causes confusion
  • index.js init() now checks the config for validity

• Tests:
  • Reordered tests in test/.
  • Input values are now in a separate file which may be of use for bulk testing

• moved the build directory from dist to build