[SPARK-1919] Fix Windows spark-shell --jars

We were trying to add `file:/C:/path/to/my.jar` to the class path. We should add `C:/path/to/my.jar` instead. Tested on Windows 8.1.

Author: Andrew Or <andrewor14@gmail.com>

Closes #2211 from andrewor14/windows-shell-jars and squashes the following commits:

262c6a2 [Andrew Or] Oops... Add the new code to the correct place
0d5a0c1 [Andrew Or] Format jar path only for adding to shell classpath
42bd626 [Andrew Or] Remove unnecessary code
0049f1b [Andrew Or] Remove embarrassing log messages
b1755a0 [Andrew Or] Format jar paths properly before adding them to the classpath