[TORQUE-997] Dependency injection: allow configuration of directories where injectors will work

Allows to customize (override) the default paths where injection scanner will look for files to inject to. Configuration is done per-application  in application descriptor. For example:

```ruby
TorqueBox.configure do
  injection do
    path "directory"
  end
end

TorqueBox.configure do
  injection do
    path ["directory1", "directory2"]
  end
end
```

I added also trace log statements.

Includes documentation and integration tests.