Move include/osmium/detail to include/osmium/index/detail.
Use -g by default for examples.
Add DynamicHandler wrapper class.

Handlers are template based and not virtual classes, so they can't be changed
at runtime. The new DynamicHandler class wraps handlers in a virtual class
hierachy and allows them to be changed at runtime for the price of some
overhead.
Rename example osmium_mpdump to osmium_area_test.
Fix some macros and includes.
Fixed include file in test.
Catch geometry errors in area_test.
Use size_t instead of int for some attributes.
Make size asked from mmap bigger in test so it will always fail.