Fix the height issue of the img tags

Fix #69.

![docs](https://cloud.githubusercontent.com/assets/1000778/5258414/c4ceb902-7a28-11e4-992d-6622d907fb0e.png)

The new img tags are generated by the following codes:

```Scala
import java.io.File
import java.net.URL
import javax.swing.ImageIcon

import scala.io._
import scala.collection.mutable

object Main {

  def imageMeta(imgURL: String): (Int, Int) = {
    val imageIcon = new ImageIcon(new URL(imgURL))
    (imageIcon.getIconWidth(), imageIcon.getIconHeight())
  }

  def clearImg(file: String): Unit = {

    val img = """<img.*src="(.*?)".*>""".r

    val urlImgCache = mutable.Map[String, String]()

    for (line <- Source.fromFile(new File(file))(Codec.UTF8).getLines()) {
      val newLine = img.findFirstMatchIn(line) map {
        imgTag => {
          val tagContent = imgTag group 0
          val url = imgTag group 1
          val newImgTag = if (urlImgCache.contains(url)) {
            urlImgCache(url)
          } else {
            val (width, height) = imageMeta(url)
            val ratio = 640.0 / width
            val scaleHeight = (height * ratio).toInt
            val newImgTag = s"""<img width="640" height="${scaleHeight}" src="${url}" alt="" />"""
            urlImgCache += url -> newImgTag
            newImgTag
          }
          line.replace(tagContent, newImgTag)
        }
      } getOrElse (line)
      println(newLine)
    }
  }

  def main(args: Array[String]): Unit = {
    clearImg("/home/zsx/workspace/RxScala/src/main/scala/rx/lang/scala/Observable.scala")
  }
}
```