TST: Improve test coverage

Add coverage for some paths that are missed in base
Merge pull request #2 from bashtage/test-arch-model

TST: Improve test coverage
Fixed the nbviewer link for examples notebook
Merge pull request #3 from hugle/master

Fixed the nbviewer link
ENH: Let hold_back and last_obs work with dates

Provide functionality so that these can accpet wither int or dates
Add testing for first_obs and last_obs
Improve appearance of plot