Allow percentage as a valid unit for height

[luckylooke]

percentage is also valid css unit for height

[coveralls]

Coverage remained the same at 70.525% when pulling 86ce62a on luckylooke:patch-2 into fa4f2d0 on troolee:develop.

[radiolips]

This seems like it would cause problems in some places. No? I know that height in percentage is really funky across different browsers.

[luckylooke]

Maybe, but should be let on user to decide, not all percentage cases are so funky ;) We are using flex and we did not catch any problems with percentage heights yet. :)

[adumesny]

actually % height makes perfect sense for nested grids where as fixed pixel doesn't!
I mean if I have a parent cell (which user can resize at will) then children probably have to take entire height and be % based. I doubt the code would allow you to add/remove rows based on drag gesture though...

[adumesny]

using #1093 as I wasn't able to merge it for some reason... thanks for the fix!