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.coffee

@@ -0,0 +1,27 @@
+Bluebird = require "bluebird"
+exec = require("child_process").execSync
+{assert} = require "chai"
+
+describe "scripts/initital-value.coffee (module.uptime(), expressed in milliseconds)", ->
+  result = exec("./test/scripts/initial-value.coffee").toString().trim()
+  it "printed #{result}", ->
+  it "printed a value above 100", -> assert.isAbove result, 100
+  it "printed a value below 350", -> assert.isBelow result, 350
+
+describe "scripts/delayed-require.coffee (sum of uptime and 250 ms delay`)", ->
+  result = exec("./test/scripts/delayed-require.coffee").toString().trim()
+  it "printed #{result}", ->
+  it "printed a value above 350", -> assert.isAbove result, 350
+  it "printed a value below 600", -> assert.isBelow result, 600
+
+describe "scripts/delayed-call.coffee (sum of uptime and 250 ms delay`)", ->
+  result = exec("./test/scripts/delayed-call.coffee").toString().trim()
+  it "printed #{result}", ->
+  it "printed a value above 350", -> assert.isAbove result, 350
+  it "printed a value below 600", -> assert.isBelow result, 600
+
+describe "scripts/difference.coffee", ->
+  result = exec("./test/scripts/difference.coffee").toString().trim()
+  it "printed #{result}", ->
+  it "printed a value above 0.005", -> assert.isAbove result, 0.005
+  it "printed a value below 0.07", -> assert.isBelow result, 0.07