Adds IE test for css expressions

[mgreter]

IE expressions are not parsed. Example is taken from:
http://msdn.microsoft.com/en-us/library/ms537634%28v=vs.85%29.aspx

Fixed by commit: mgreter/libsass@691e101
Pending in pull request sass/libsass#181

Tested via CSS::Sass mgreter/perl-libsass@91884f0

[nex3]

For reference, the reference implementation parses expression(...) as a form of string, much like url(...) and calc(...).