Create artifacts in batches

We've got a significant number of small build artifacts. We're frequently seeing:

```
--- Uploading artifacts
2014-06-23 14:05:48 [INFO ] Found 893 files that match "results/**/*; log/*; tmp/capybara/**/*"
2014-06-23 14:06:47 [FATAL] Failed to upload artifacts: Unexpected error: 504 Gateway Timeout
```

This seems to be due to timing out when creating the artifacts on buildbox.

Splitting the requests into 100 files a piece seems like a reasonable workaround.
I wanted to use some fancy iterators, or at least `artifacts[i::100]` or something but couldn't figure it out.