Added ./u entries in .gitignore

This adds two entries in the .gitignore file:

    ./u
    ./u/*

for ignoring such a subdirectory. The gentle user is now able to put
her/his utility scripts in there.

For example, one could put an astyle helper (callastyle.sh) in there:

    #!/bin/bash
    astyle --options=./etc/astyle

as typing the astyle call by hand can be annoying.
Added entry for ./u in CONTRIBUTING.md
Add prequisites for graph inclusion
Ignore metapost output
Add section about operations
Add prelimary section on AVL internal operations
Add subsections about AVL operations
Add graph visualizing right-rotations
Add subsection about rebalancing subtrees
Merge pull request #37 from neithernut/add-paper-operations

Add paper operations
Merge pull request #33 from matthiasbeyer/add-gitignore_util_ignore

Added ./u entries in .gitignore