Optimize ScalarBaseMult

Code uses the binomial expansion strategy which eliminates doublings.
All the basepoint doublings are computed beforehand to assist in the base multiplication.
Main function is computeDoublingPoints which computes all the doublings of the generator point G up to the size of the curve's bit size (for secp256k1, that's 256).

Also fixed a spelling error in a benchmark test.

Results so far seem to indicate the time taken is about half what it was before.

Closes #2
This should be:
```Go
// Copyright (c) 2014 Conformal Systems LLC.
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.
```

Since the file wasn't created by any of the others unlike the `btcec.go` which was originally based on the others.  Although at this point, it's basically completely different anyways.