API accepts a public key string or an array

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