Merge changes upstream

These changes made it work well with Revel. Mostly: 

* Allow merging of config files
* Make it possible to test for property retrieval errors 
* Export the "DEFAULT_SECTION" identifier
* Make getters fall back to DEFAULT - ddb9a3edc5f0041369be3b875929452dcc4659e3

The last one is potentially backwards incompatible, but it fixed an inconsistency in my opinion.