Added issue 16 to CHANGES
Support for @import directive

The @import directive allows importing a definition file inside another.
The syntax is similar to @import in css.

    # Same folder
    @import constants_en.txt
    # Parent folder
    @import ../constants_en.txt
    # Absolute path
    @import /path/to/constants_en.txt

Semantics:
- The definitions in the imported filed are added to the registry
  when the @import statement is found
- Path are written in unix style

In addition, the default file is is provided to load_definitions using
pkg_resources.resource_filename instead of pkg_resources.resource_string
to allow for relative import of constants.

closes #22