由于svg控制中只有通过text和tspan两个标签进行操作，所以几乎所有的对齐方式都得通过计算来获得，找了很久找到这篇帖子http://bl.ocks.org/MSCAU/58bba77cdcae42fc2f44。
里面介绍了几个很有用的属性可以操作：
1 .getBBox().width
IE and Firefox can't get the BBox of a TSPAN and error out.
2.getBoundingClientRect().width
Integer in IE, Chrome and Safari. Same width as "test1" in Chrome eventhough 2 is wider than 1.
3 .getComputedTextLength()
A TSPAN which is not in a TEXT has no width
