Add space to separate existing filters from alpha filter

In IE <= 8 it was impossible to setOpacity on an element that already had an existing filter such as filter: FlipH; The filters need to be space seperated
Fix regression with handling of `$break` inside `Array#inject`. [close #162]
Fix error in `Element.positionedOffset` that made incorrect adjustments for margins. [close #141]
Fix issue where `getOpacity` would return a string, not a number, in newer versions of IE.

Also fix documentation that incorrectly claimed `getOpacity` was designed to return a string.

[close #116]
Merge pull request #107 from superdav42/master

Add a space when we apply the `alpha` filter in old versions of IE so that we don't disrupt existing filters.
Merge branch 'master' of github.com:sstephenson/prototype
Fix regression in behavior of `someElements.each(Element.toggle)`. [close #136]
Add documentation to explain how `HTMLSelectElement#remove` clashes with `Element#remove`. [close #122]