Various minor fixes

1. AeroGearCrypto is missing private constructor - https://issues.jboss.org/browse/AGSEC-157
2. removed unused import from CryptoBoxTest
3. Improved checkLenght method in case it takes null - https://issues.jboss.org/browse/AGSEC-158
4. added test for 3
5. repaired minor spelling issues in README.md
6. hardened newByteArray - https://issues.jboss.org/browse/AGSEC-159
7. added test for 6
8. extracted activity class name in Util static block to constant

It seems to me there should be some related security discussion how to code more defensively when we code security tooling.