Header overhaul

[stonier]

• Drop "../../" relative includes, some will be reaching into binary directories anyway
• Drop global include_directories settings
• Take advantage of target interface settings for build/install
• Dump headers over in the include directory, take advantage of delphyne/... (see conversation in [infra] protobuf -> src/protobuf #407)?

[stonier]

GSG:

All of a project's header files should be listed as descendants of the project's source directory without use of UNIX directory shortcuts . (the current directory) or .. (the parent directory). For example, google-awesome-project/src/base/logging.h should be included as:

#include "base/logging.h"

GSG/cpplint is quite a beast. So many rules it's like learning a new language and more than a few don't seem that compelling nor do they fit all use cases.

[stonier]

Remaining work subsumed by #472.