Some text goodies

Ok basic support for em unit.
Also better handling of font declaration in global style, and remove the svg declaration from style that was confusing our style parsing logic.

Not perfect but going better:

now correct style is parsed (font family and font size ) and correct height of text is used.

![image](https://cloud.githubusercontent.com/assets/1194048/4784295/9732a770-5d4e-11e4-9f61-31faba00f5bb.png)