js: make method parameters objects

• As requested by using parties, the options to be passed into the methods are now objects, with the objects' properties better describing what they do, and to avoid the need to type several nulls in a method call if one wants the last parameter.
  • src/Keyring.js, src/gpgme.js: Changed parameters and their validations
  • BrowserTest/*.js Had to adapt quite some calls to the new format