class Key:
Added support for reading Private and Public Keys in PEM format. So you can create keys using OpenSSL:
	openssl ecparam -out rawkey.pem -name secp256k1 -noout -genkey
and protected by a password (using aes-256-cbc encryption) by:
	openssl ec -in rawkey.pem -out key.prv -aes-256-cbc
Finally, public keys can be extracted using:
	openssl ec -in key.prv -out key.pub -pubout

class Client:
Added a cancel method to stop any on going calls.