Correct delta()/rate() intervals and temporal aliasing.

This addresses https://github.com/prometheus/prometheus/issues/137.

Before:
![aliased_rate](https://f.cloud.github.com/assets/538008/375840/a8e342ee-a3d3-11e2-9951-d7f62c4f8a7f.png)

After:
![corrected_rate](https://f.cloud.github.com/assets/538008/375842/b25dd73a-a3d3-11e2-9cfb-04f817d0f478.png)