Display abandonded availabilities on the weekly metrics report

I'm interested in seeing if mentors might be offering less availabilities after availabilities they offer are unused. This pull request adds "abandoned availabilities" (unused availabilities that are past the time when they can be used) data to the weekly report.

Screenshot of live data from a few weeks ago:

![screenshot 2014-03-19 11 16 13](https://f.cloud.github.com/assets/202/2462676/bd271642-af83-11e3-81b8-b0b95587a54b.png)

Query from live database that backs it up:

```
Availability.abandoned.where(:start_time => (a..b)).order("start_time ASC").map { |av| "#{av.mentor.name} #{av.local_start_time}" }
=> ["Rich Vogt 2014-03-04 17:15:00 -0600",
 "Andrew Jorczak 2014-03-05 17:30:00 -0600",
 "Andrew Jorczak 2014-03-05 22:30:00 -0600",
 "Lucas Willett 2014-03-07 17:30:00 -0600"]
```