Change loading schema.

1. For local files use require_relative instead of require. For files
from other gems still use require. require_relative safer than require,
because it don't go through global paths.
So we can safely use monkey patching in our internal project.

2. Change hierarhy loading to load all file on top level. Use
require_relative in 3 top files: openehr.rb, rm.rb and am.rb.
So we can delete require at the end of some files. For example, in
item_structure.rb, content.rb.

3. Use require_relative in head of code for dependecies between files.

4. Delete modification of $:, unnecessary include, use long names of classes.

5. Delete files (not top level) with only require, for example support.rb

This makes code more clean and readable.