Add a navigationType property to the Metric object

[philipwalton]

Fixes #214.

This PR adds the navigationType property to the Metric object as sets its value based on the following logic:

• If the page was loaded via a normal navigation and the browser supports the Navigation Timing API, the type value from the navigation entry is used.
• If the page was loaded via a normal navigation but the browser does not support the Navigation Timing API, the value will be undefined.
• If the page was restored from the bfcache, this value will be 'back_forward_cache' (not to be confused with a value of 'back_forward' from the Navigation Timing API after a normal back/forward navigation).

Note: In the future, this property could potentially include other values, e.g. a soft navigation in an single-page application, but currently only the above are supported.