rename irony-{clang-working-directory => compiler-invocation-directory}
CDB: small comments/documentation bump
CDB: privatize some functions, minor tweaks
CDB: start fresh with a better, simpler design and interface
CDB: keep track of the current completion database

Display the the current compilation database and the compile options in
CDB menus. Ugly right now but can be enhanced in the future.
CDB: put the compilation database in its own file

This allows nicer private prefixes irony-cdb-clang-complete--<xxx>
instead of irony-cdb--clang-complete-<xxx>.

When customize variables and other configurations will be added to the
compilation databases we will be able have a better customize group.
CDB: initial support for JSON Compilation Database