Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/deep-is/example/cmp.js

@@ -0,0 +1,11 @@
+var equal = require('../');
+console.dir([
+    equal(
+        { a : [ 2, 3 ], b : [ 4 ] },
+        { a : [ 2, 3 ], b : [ 4 ] }
+    ),
+    equal(
+        { x : 5, y : [6] },
+        { x : 5, y : 6 }
+    )
+]);