Adds a switch to get units in rems instead of ems

Moved all functions from _global.scss to _functions.scss because they
were being duplicated in _variables.scss. This also solves the problem
of using these functions in local variables overrides as you only need
to include the functions.

Changed all hardcoded 'em' values to use emCalc.

Added a `$use-rems` variable which is true by default.