Implement 'vertical-align' property.

It implements 'vertical-align' property and adds a simple test case.

However, there are several things to be fixed.
- We should calculate top and bottom value for margin, border, and padding in advance.
- For the 'middle' value of vertical-align, we should know 'x-height' of font.
- For the 'sub' and 'super' value of vertical-align, the proper position should be chosen.