Merge pull request #58 from jrozner/master

Changes for doing a proper system install and Go bindings
Allow in-place build via scons --in-place
Make install_name darwin-only
Merge pull request #61 from thequux/install_name-is-darwin-only

Make install_name darwin-only
Merge pull request #60 from thequux/allow-in-place-build

Allow in-place build via scons --in-place
Added a void* user_data pointer to HAction
Added a void* user_data pointer to HPredicate
Added documentation for H_{A,V,VA,AV}DRULE
Merge pull request #62 from thequux/add-callback-userdata

Add void* userdata to callbacks