js: more Keyring/Key handling

• src/Keys.js
  • made setKeyData more consistent with other methods
  • added convenience methods (Key.armored, Key.hasSecret)
  • Added a Key delete function

• src/Keyring.js:
  • added a getkeysArmored which allows for bulk export of public Keys

gpgmejs:

• removed deleteKey. It is now a method of the Key itself
• Encrypt: Added some common options as parameter, and the possibility to set all allowed flags via an additional Object