Handle Unicode whitespace and invisible characters

Unicode defines a number of whitespace and invisible characters that strip_attributes does not currently strip. This pull request adds support for stripping these.

I created the list based off of [this](http://www.cs.tut.fi/~jkorpela/chars/spaces.html) and [this](http://unicode.org/Public/UNIDATA/PropList.txt). I'm not a Unicode expert so it's possible I missed some or included some I shouldn't have.