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/browser-process-hrtime/README.md

@@ -0,0 +1,25 @@
+# browser-process-hrtime
+
+Browser shim for Node.js process.hrtime().
+See [documentation at nodejs.org](http://nodejs.org/api/process.html#process_process_hrtime)
+
+### usage
+Use hrtime independant of environment (node or browser).
+It will use process.hrtime first and fallback if not present.
+```js
+var hrtime = require('browser-process-hrtime')
+var start = hrtime()
+// ...
+var delta = hrtime(start)
+```
+
+### monkey-patching
+You can monkey-patch process.hrtime for your dependency graph like this:
+```js
+process.hrtime = require('browser-process-hrtime')
+var coolTool = require('module-that-uses-hrtime-somewhere-in-its-depths')
+```
+
+### note
+This was originally pull-requested against [node-process](https://github.com/defunctzombie/node-process),
+but they are trying to stay lean.