Fix mathematical operations with different units

[mgreter]

Fix mathematical operations with different units:

foo { bar: (1cm*2in/1cm); }

Could move convert function back to normalize, but I don't think we pay much in terms of performance for keeping them apart for now. Made quite a few wrong turns while implementing that, since I intially wanted to use a map table for the units (like counting exponents). But then it was not able to normalize units the way ruby sass does, as insert order is important for that.

So this is not just a cosmetic fix, libsass also calculated wrong results in certain situations (see changes in eval.cpp). I do hope this is on par with ruby sass now 😉

Makes it pass this spec test:

• libsass/rel
• libsass/list
• libsass/conversions

IMO it's only missing (optional) performance optimizations!

[coveralls]

Coverage increased (+0.0%) to 80.56% when pulling eed6698 on mgreter:bugfix/math-ops-with-units into f82a41b on sass:master.

[coveralls]

Coverage increased (+0.02%) to 80.28% when pulling b5ac186 on mgreter:bugfix/math-ops-with-units into 9ad775f on sass:master.