Create / demonstrate in tests method for recovering address from signTypedData

[thegeronimo]

Issue_title

There doesn't seem to be a clear way to verify the output of signTypedData.

Issue_description

Recovering the public address from a signature and message from signTypedData is not clear. While there is a test for signedTypedData there is not a test for recovering the public key. Such a test would serve as good documentation for how it can be done using the library and will also serve to make the codebase more robust.

Issue_context

signedTypedData definition: https://github.com/web3j/web3j/blob/master/crypto/src/main/java/org/web3j/crypto/Sign.java#L85

testSignedTypedData: https://github.com/web3j/web3j/blob/master/crypto/src/test/java/org/web3j/crypto/SignTest.java#L87