[SPARK-1870][branch-0.9] Jars added by sc.addJar are not in the default classLoader in executor for YARN

This summary is copied from Sandy's comment in the mailing list.

The relevant difference between YARN and standalone is that, on YARN, the 
app jar is loaded by the system classloader instead of Spark's custom URL
classloader.

On YARN, the system classloader knows about [the classes in the spark jars,
the classes in the primary app jar].   The custom classloader knows about
[the classes in secondary app jars] and has the system classloader as its
parent.

A few relevant facts (mostly redundant with what Sean pointed out):
* Every class has a classloader that loaded it.
* When an object of class B is instantiated inside of class A, the
classloader used for loading B is the classloader that was used for loading A.
* When a classloader fails to load a class, it lets its parent classloader
try.  If its parent succeeds, its parent becomes the "classloader that
loaded it".

So suppose class B is in a secondary app jar and class A is in the primary
app jar:
1. The custom classloader will try to load class A.
2. It will fail, because it only knows about the secondary jars.
3. It will delegate to its parent, the system classloader.
4. The system classloader will succeed, because it knows about the primary
app jar.
5. A's classloader will be the system classloader.
6. A tries to instantiate an instance of class B.
7. B will be loaded with A's classloader, which is the system classloader.
8. Loading B will fail, because A's classloader, which is the system
classloader, doesn't know about the secondary app jars.

In Spark standalone, A and B are both loaded by the custom classloader, so
this issue doesn't come up.

In this PR, we don't use customClassLoader anymore. We add URL to the 
current classloader instead. Since AddURL is protected method in URLClassLoader,
calling the protected method is achieved through reflection.