[SPARK-9012] [WEBUI] Escape Accumulators in the task table

If running the following codes, the task table will be broken because accumulators aren't escaped.
```
val a = sc.accumulator(1, "<table>")
sc.parallelize(1 to 10).foreach(i => a += i)
```

Before this fix,

<img width="1348" alt="screen shot 2015-07-13 at 8 02 44 pm" src="https://cloud.githubusercontent.com/assets/1000778/8649295/b17c491e-299b-11e5-97ee-4e6a64074c4f.png">

After this fix,

<img width="1355" alt="screen shot 2015-07-13 at 8 14 32 pm" src="https://cloud.githubusercontent.com/assets/1000778/8649337/f9e9c9ec-299b-11e5-927e-35c0a2f897f5.png">

Author: zsxwing <zsxwing@gmail.com>

Closes #7369 from zsxwing/SPARK-9012 and squashes the following commits:

a83c9b6 [zsxwing] Escape Accumulators in the task table