This commit fixes the behavior of (BitCurve).Add() to be more inline
with btcd. It changes two different bugs that occured.

1) When adding a point at infinity to another point, the other point
should be returned. While this is undefined behavior, it is better
to be more inline with the go standard library.
Thus (0,0) + (a, b) = (a,b)

2) Adding the same point to itself produced the point at infinity.
This is incorrect, now doubleJacobian is used to correctly calculate it.
This is also similar to the go standard library.
Thus (a,b) + (a,b) == 2* (a,b) and not (0,0) anymore.