Add optional callback function for full buffer.
Improved and extended equality and less-than operators.
Add iterator interface to osmium::io::Reader class.

This makes it very easy to access the content of OSM files with STL algorithms
etc. All the file and buffer handling is hidden behind a simple input iterator
interface.
Remove misleading comment.
Object comparison improvements.

They are now in their own file and the functors also support comparison of OSM
object pointers, not only references.
Bugfix: Use std::abs() instead of abs()

The C abs() is not overloaded for types other than int, but std::abs() has
overloads for int, long int, and long long int.
Improve code for comparisons between OSM objects.
Simplify sort of object ptr collection