Add decrypt function.

Decrypt is used throught an annotate class which creates an alias to
access the decrypted value.
Update README.md [ci-skip]
Update CHANGELOG.md [ci-skip]
Update README and aggregates docstring [ci-skip]
Refactor test_value
Update docstrings for `DecryptFunctionSQL` and `Decrypt`.
Merge pull request #3 from incuna/decrypt-value

Add ability to decrypt fields