Fix ECPointFp.decodeFrom with compressed keys

Added test case for compressed and uncompressed keys, and made it work with compressed keys based on code from vbuterin's bitcoinjs-lib.

This is what was causing `ecdsa.verify()` to fail with compressed keys.

One thing I wasn't sure about - in the original code you had this:

```js
  // Prepend zero byte to prevent interpretation as negative integer
  xBa.unshift(0);
  yBa.unshift(0);
```

Which was removed by vbuterin on bitcoinjs-lib. I removed it as well. Is that needed?