Add e(), log(), pow() and sqrt() functions

[efoken]

Add some missing math functions from Compass.

https://github.com/chriseppstein/compass/blob/stable/lib/compass/sass_extensions/functions/math.rb
http://compass-style.org/reference/compass/helpers/math/

[eevee]

Done.

My implementation is slightly different from theirs, though: I also raise the units to the given power. I argue that this is correct, though I also argue that raising units to a power is a bit bozotic.