string_ref and external boost 1.53 is no good

@c42f For some reason @gadomski is getting an error when he compiles against an external boost 1.53.  Should we just include string_ref into PDAL for now and hide it behind the pdal:: namespace? It rather significantly increases our boost requirement, and doesn't seem so stable version-to-version as far as boost releases goes right now...


```
/home/gadomski/Code/PDAL-rivlib/test/unit/drivers/rxp/TestReader.cpp:33: undefined reference to `pdal::Schema::getDimension(boost::basic_string_ref<char, std::char_traits<char> >, boost::basic_string_ref<char, std::char_traits<char> >) const'
```

The offending code looks like this:

```
pdal::Dimension const& dimX = schema.getDimension("X");

```