Add more crypto backends to keepass.io

[ppmathis]

More crypto backends could be added to keepass.io, example given (sorted based on priority, from high to low):

-> Multithreaded C++ cryptography backend (as requested in issue #13)
-> Web Cryptography API (for use within browserify)
-> Chrome Packaged App library (PNaCl)

The existing core of the library should be expanded so that it easily supports more cryptography backends in the future. None of those new backends must be required to use keepass.io