Tachyon-74 : Add free of in-memory file/folder from CLI

Add a new cli to free in memory file/folders
@haoyuan : Unit test is failing when I change to assertFalse. I am trying to figure out why? 
When I test manually, it seems to work fine though.
Here is the output of unit test:
Tests run: 29, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 359.928 sec <<< FAILURE!
freeTest(tachyon.command.TFsShellTest)  Time elapsed: 12.376 sec  <<< FAILURE!
java.lang.AssertionError: null
        at org.junit.Assert.fail(Assert.java:92)
        at org.junit.Assert.assertTrue(Assert.java:43)
        at org.junit.Assert.assertFalse(Assert.java:68)
        at org.junit.Assert.assertFalse(Assert.java:79)
        at tachyon.command.TFsShellTest.freeTest(TFsShellTest.java:512)