Add benchs

Hi. I added [ecdsa](https://github.com/cryptocoinjs/ecdsa) and [secp256k1](https://github.com/wanderer/secp256k1-node) to the benchmark.

Results on my system:
```
Benchmarking: sign
elliptic#sign x 292 ops/sec ±0.73% (177 runs sampled)
sjcl#sign x 74.50 ops/sec ±0.40% (153 runs sampled)
openssl#sign x 2,145 ops/sec ±0.22% (198 runs sampled)
ecdsa#sign x 45.28 ops/sec ±0.15% (117 runs sampled)
secp256k1#sign x 9,598 ops/sec ±1.65% (191 runs sampled)
------------------------
Fastest is secp256k1#sign
========================
Benchmarking: verify
elliptic#verify x 133 ops/sec ±0.69% (170 runs sampled)
sjcl#verify x 63.09 ops/sec ±0.37% (130 runs sampled)
openssl#verify x 1,955 ops/sec ±0.06% (199 runs sampled)
ecdsa#verify x 32.84 ops/sec ±0.42% (114 runs sampled)
secp256k1#verify x 10,370 ops/sec ±0.03% (202 runs sampled)
------------------------
Fastest is secp256k1#verify
========================
Benchmarking: gen
elliptic#gen x 321 ops/sec ±0.45% (182 runs sampled)
sjcl#gen x 82.28 ops/sec ±0.07% (141 runs sampled)
------------------------
Fastest is elliptic#gen
========================
Benchmarking: ecdh
elliptic#ecdh x 165 ops/sec ±0.80% (169 runs sampled)
------------------------
Fastest is elliptic#ecdh
========================
Benchmarking: curve25519
elliptic#curve25519 x 75.02 ops/sec ±2.64% (115 runs sampled)
jodid#curve25519 x 107 ops/sec ±0.30% (156 runs sampled)
------------------------
Fastest is jodid#curve25519
========================
```