Change error message: Not necessarily an "io" error.
Add geojson test.
Take apply() call out of Reader class and make it a top-level function.
Make apply() work on node::Buffer's.
Put apply() code in its own file.
Fix location of data file in test.
Support for reading OSM file from node.Buffer.

An osmium.File can be opened now not only with a filename, but also with a
node.Buffer containing the data as it would be in the file. Closes #16.