Add per function callbacks (closes #26)
Move return value in after callback signature
Use move to save std::functions
Merge branch 'master' into per_function_callbacks
Reduce memory footprint for compilation with reducing the number of callbacks to one
Merge branch 'master' into per_function_callbacks
Update README.md
Replace explicit std::funtion validity flag with usage of nullptr
Merge branch 'master' into per_function_callbacks

Conflicts:
	README.md
Merge pull request #54 from hpicgs/per_function_callbacks

Per function callbacks