Switching random generator

[bcleenders]

Maybe using math/rand is not the best idea for creating "random" secrets. Instead, you should probably use crypto/rand (which provides cryptographically random data).

I forked your project to use crypto/rand instead: https://github.com/bcleenders/gototp

I didn't do a pull request, because I changed your interface (since users no longer give a random generator as argument). Thus, if anyone is using your code they would get errors when rebuilding.

Feel free to pull if you don't think that's a problem, though...

PS: still nice to have your code available!