API accepts a single public key or an array

Changes the API to allow a single public key object to be sent, instead of as a one-element array.

Where currently you must send ```encryptMessage([PublicKey],PlainText)``` you can also send ```encryptMessage(PublicKey,PlainText)```.

An array of public keys can still be sent, but this clears up a confusing requirement responsible for developer errors (see #189 and #194)