Removal of secure-random in favour of crypto-browserify

ecurve was bumped to `1.0.0`, no reason not to use that, given the only changes since `0.10.0` was removed support for the `secp224r1` curve, and a typo.

Now using `crypto-browserify` 3.0.0 so we can drop another dependency (`secure-random`). 

Not a big fan of the name here. Seems like crypto2 is only used once so why not `var sha256 = require('../src/crypto').sha256`?