Make it easier to compare multiple versions of a library

```
Sometimes I want to benchmark two versions of the same library against
eachother.

For example:
- Arrays.sort between jdk6 and jdk7
- MapMaker between google collections 1.0 and guava r08.

And it's a huge pain in the bum.

Can caliper help? It seems a bit hopeless at first. But....

We could have a restriction that from the perspective of the benchmark, each
version of the lib has the same API. Then, the benchmark class is annotated
with the list of versions to compare and the (roughly) version->classpath
mapping is provided at runtime. Caliper can now swap the classpath for me!

Other thoughts:
- Maybe this becomes easier with java 7.
- These gymnastics don't belong in caliper, it can be done from a wrapper lib.

What do you think?
```

Original issue reported on code.google.com by `brianfromoregon` on 20 Dec 2010 at 8:03