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/performance-now/test/scripts/initial-value.coffee

@@ -0,0 +1,10 @@
+#!/usr/bin/env ./node_modules/.bin/coffee
+
+###
+Expected output is a number above 100 and below 350.
+The time reported is relative to the time the node.js process was started
+this is approximately at `(Date.now() process.uptime() * 1000)`
+###
+
+now = require '../../lib/performance-now'
+console.log now().toFixed 3