Added verbose logger.

Between two machines `unix_instruments` hung in two different ways (one with `/dev/ttyvf` being busy, another without an error). The first one cleared after a reboot, working on the other one. In either case bwoken gave no indication of what's going on.

This PR adds a logger that you can call from anywhere, for now it just logs the commands being run, so one of my outputs is stuck with this, at least this is something one can rerun manually to figure out where the issue is.

```
D, [2013-12-19T18:07:47.706289 #4615] DEBUG -- : "/Users/dblock/.rvm/gems/ruby-2.0.0-p353/bundler/gems/bwoken-e7d6ffbe5005/bin/unix_instruments.sh"                  -D "/Users/dblock/source/artsy-ios-foo/dblock/integration/tmp/trace"         -t "/Applications/Xcode.app/Contents/Applications/Instruments.app/Contents/PlugIns/AutomationInstrument.bundle/Contents/Resources/Automation.tracetemplate"         "/Users/dblock/source/artsy-ios-foo/dblock/build/iphonesimulator/Foo.app"         -e UIASCRIPT "/Users/dblock/source/artsy-ios-foo/dblock/integration/tmp/javascript/iphone/test_shows_list.js" -e UIARESULTSPATH "/Users/dblock/source/artsy-ios-foo/dblock/integration/tmp/results"
```