Fix small issues with commit 680d3188

Properly document the SLIDEATLAS_GOOGLE_* config variables.

Makefiles are required to use tabs, so specify this fact in the
gitattributes.
Specify an equivalant tab width of 4 spaces in gitattributes

The default is 8 spaces. Setting it properly to 4 allows for the correct
detection of "indent-with-non-tab" errors and auto-correction.
Merge branch 'test/smoke_test'

* test/smoke_test:
  Specify an equivalant tab width of 4 spaces in gitattributes
  Fix small issues with commit 680d3188