Added Bouncy Castle cryptographic provider to build.gradle
Switched cryptographic provider to Bouncy Castle
Switched RSA signing algorithm to RSASSA-PSS
Merge pull request #63 from L-Henke/bouncyCastle

Added Bouncy Castle cryptographic provider to build.gradle
Changed rsaSign() parameter name from 'message' to 'data' to avoid confusion
Changed rsaSign() Javadoc description.
Renamed encryptHybrid() to encryptHybridAndSign() and
decryptHybrid() to decryptHybridAndValidateSignature() because encryption
and signing are indepented operations. This should be expressed by the
method name.