Allow dynamic detection of decryption keys

[zarqman]

This breaks apart #7, as requested.

This is part 2, which allows a block to be given to dynamically determine what key to use during decryption. The block is given the headers, allowing use of whatever key-identifying header may have been set during encryption.

This implementation is intentionally similar to dynamic key detection already present in ruby-jwt, making it easy and fluid to move between the two.