0.2.0 with minor enhancements

Thanks to Paul Harris, version 0.2.0 of libkdtree++ features some minor enhancements to the code. The most important are:

- a full set of range constructors and insert(range) methods. It now works with inserter(tree,tree.begin()).
- Target type no longer needs a default constructor. - Added a new method: count_within_range().

Thanks, Paul, and thank you for using libkdtree++!