Adding sass variables for footer, subfooter and subheader heights
Allowing to footers and subheader with diferent height.

FIX #2334
adding tests for both subheaders and subfooters
Merge branch 'variable-subheader-height' of https://github.com/arturokunder/ionic into arturokunder-variable-subheader-height
Merge branch 'arturokunder-variable-subheader-height'
style(): set $bar-height to $bar-subheader-height
Merge branch 'master' of https://github.com/driftyco/ionic