[WIP] Add some security utilities

This adds some security utilities to core including:
- A library for basic crypto operations
 - This is especially useful because now a lot of our application use their own crypto for some operations (e.g. obfuscate passwords), while a central solution should be obviously preferred
 - ` $encryptWithDefaultPassword = \OC::$server->getCrypto()->encrypt('EncryptedText');`
 - `$encryptWithCustomPassword = \OC::$server->getCrypto()->encrypt('EncryptedText', 'password');`
- A better library for random numbers
 - Mixes several sources as specified in [RFC 4086 Section 5.2](http://tools.ietf.org/html/rfc4086#section-5.2)
 - Supports different strengths. Specifically: low and medium
    -  `\OC::$server->getSecureRandom()->getLowStrengthGenerator()->generate(10);`
    -  `\OC::$server->getSecureRandom()->getMediumStrengthGenerator()->generate(10);`
 - Allows to specify a charset
    -  `\OC::$server->getSecureRandom()->getLowStrengthGenerator()->generate(10, 'abc');`
- A library for secure "somewhat constant" string comparisons (`\OCP\Security\StringUtils::equals($expected, $input);`)
 - This is helpful against Timing Attacks. We have some workarounds in the code where we do this manually, but we should solve it centrally.

@DeepDiver1975 @karlitschek 
Document that this uses Encrypt-then-MAC (EtM)