Update lib/compass/sass_extensions/functions/math.rb

`Sass::Script::Functions.declare :pi, []` was declared twice. I believe this should be declaring e, as it's right after e.
Merge pull request #1124 from Snugug/patch-1

Update lib/compass/sass_extensions/functions/math.rb
Added column-span
Merge pull request #1132 from adius/stable

Added column-span
merge stable from chriseppstein/stable