add very basic implementation of opposite-position function

this is implemented in compass as ruby code
Merge pull request #11 from koala-framework/master

add very basic implementation of opposite-position function
add elements-of-type function.
Merge pull request #12 from hirayama-evolni/add-elements-of-type

add elements-of-type function.
Update README.markdown
Merge remote-tracking branch 'Igosuki/master'

Conflicts:
	lib/compass/_functions.scss