Dev tool: scopeHierarchyToString

Introducing MortarScopeDevHelper.scopeHierarchyToString() to convert any mortar scope into a human readable string representation of its hierarchy.

Example:

```
Root
|-com.squareup.LoggedIn
| |-com.squareup.MyActivity
| | |-HOME
| |-com.squareup.MyOtherActivity
| |-com.squareup.YetAnotherActivity
| | |-SomeParentScope
|-com.squareup.ui.StartActivity
```
Merge pull request #15 from square/py/scope_hierarchy_to_string

Dev tool: scopeHierarchyToString
Nice, thanks.