Add support for ERXCryptoString to ERRest when the content type is JSON

This change adds the required code to transform ERXCryptoString fields into JSON format and to transform the corresponding text from JSON format into an ERXCryptoString object.
Merge pull request #519 from hprange/erxCryptoStringERRestSupport

Add support for ERXCryptoString to ERRest when the content type is JSON