new reporter system

@yiding This is the new reporter scheme we discussed.  

Reporters are their own executables that read JSON events from STDIN and emit formatted results to STDOUT.

This means people are free to write their own reporters without modifying _xctool_ itself.  Reporters don't even need to be written in ObjC anymore.

You can even use something like __/bin/cat__ as a reporter - it'll just emit every JSON event it receives and be equivalent to the __json-stream__ reporter.

```
xctool \
  -project xctool/xctool-tests/TestData/TestProject-App-OSX/TestProject-App-OSX.xcodeproj \
  -scheme TestProject-App-OSX \
  test \
  -reporter /bin/cat
```

All of the existing Reporter implementations have been moved into their own 'reporters' project.  They're installed at `path/to/xctool/reporters`.