Add the Footprint instrument

```
= What steps will reproduce the problem?
1. Declare method with @Footprint annotation
2. There is no notable difference with using that annotation

= What is the expected output? What do you see instead?
Expected to see memory footprint of the returned object.

=What version of the product are you using? On what operating system?
com.google.caliper 1.0-beta-SNAPSHOT

= Please provide any additional information below.
As I see, there is no processing of the @Footprint at the moment, while
configuration mentions
"instrument.footprint.class=com.google.caliper.runner.ObjectFootprintInstrument"
,
there is no ObjectFootprintInstrument class in the source control.

Please close if it not an occasion and footprint instrument is intentionally
not published.

```

Original issue reported on code.google.com by `cee...@gmail.com` on 30 Jul 2013 at 6:01