js: change the write access for js class methods

• src/ [Connection, Error, Key, Keyring, MEssage, Signature, gpgmejs]: Functions and values that are not meant to be overwritten are now moved into their constructors, thus eliminating the possibility of overwrites after initialization.

• Key: The mode of use (synchronous cached, or async promises) ivs now determined at initialization of that Key. The property Key.isAsync reflects this state.

• unittests: fixed old Key syntax for testing.

• Message.js isComplete is now a method and not a getter anymore.

• Added some startup tests.