Localize path expansions to helper method

Hi, 

This changeset reduces the duplication of File.expand_path and the state of that expansion to a single app_path method in tracker. This simplifies extensions that do not rely on path based AppTrees (i.e., remote git or http endpoints). 

One possible downside to this change is in the output messages on the console (not in the reporting). For example (from https://github.com/codeclimate/brakeman/blob/master/lib/brakeman.rb#L265):

```
notify "Processing application in #{options[:app_path]}"
tracker = scanner.process
```

In this case the output will not show a fully expanded path and will instead show the literal path that was submitted. Again, in the case of non-file-based scans this is a benefit.
This should be cached since it should never change.