promise version of co

quickly hacked together a promised version of co using bluebird. benchmarks for callback-based co:

```bash
          31,859 op/s » promises
          48,958 op/s » thunks
          30,536 op/s » arrays
          33,941 op/s » objects
          26,751 op/s » generators
          50,286 op/s » generators delegated
          26,836 op/s » generator functions
```

results for this promised based:

```bash
          44,526 op/s » promises
          43,249 op/s » thunks
          24,943 op/s » arrays
          26,247 op/s » objects
          37,559 op/s » generators
          50,534 op/s » generators delegated
          37,982 op/s » generator functions
```

a little slower, but there's room to improve. best of all, though, the code is much simpler since we can remove most of the try/catches