Simplified Application._executeStartupFiles().

IECore.ConfigLoader provides a subdirectory argument now, so there's no need to manipulate the paths.
Added support for module level config files.

These sit alongside the application config files on the GAFFER_STARTUP_PATHS, but are in subdirectories by module name rather than application name. They provide a convenient place to register functionality that will be available across all applications and scripts which use a particular module.
Merge pull request #945 from johnhaddon/moduleConfigs

Module level config files