Refactor, rename
Update Nexus5 template
Add Nexus6 template
Add Legacy HTCG2 template
handle overlay.png layer, raise new LayerNameError
Update NexusOne.xcf with overlay.png
Merge pull request #5 from ncornette/templates

Templates Updates:
  - Updated Nexus 5 template with new picture
  - Added Nexus 6 template
  - Added Legacy HTC G2 template
  - Refactor