Remove using namespace v8.
Refactored SET_ACCESSOR() macro into set_accessor() function.
Use v8::String::NewSymbol() instead of New() in a few places.

This should make things slightly faster.
Open and close HandleScope correctly everwhere.
Reorganize access to timestamp attribute of OSM objects.
Make Node/Way/RelationWrapper really inherit from OSMObjectWrapper.
Make some functions private.
Wrap wrapper object creation in static create() function.
Make test target depend node.osmium.
Add tests showing OSM objects can't be created from Javascript.
Tighten New function of OSM object wrappers.
Simplify setting of timestamp() function.

This is possible now that Node, Way, and Relation inherit from OSMObject.
Replace macro NODE_SET_PROTOTYPE_METHOD by function call.
Reorganize OSM object wrapper .hpp and .cpp files.
User const osmium::OSMEntity not an iterator as base for OSM object wrappers.
Cleanup and test tag access functions.
Hide unused parameter.
Simplify code.
Check whether argument to tags() function is a String.
Cleanup nodes() function and test it. Add nodes_count property.