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
Added files for linkedlist implementation
We insert at the head of the linkedlist
When inserting at the head, we shouldnt call this append
Added implementation for ll_insert()
Added implementation for ll_insert_data()
Added implementation for ll_element_alloc()
Added implementation for ll_element_alloc_new()
Added implementation for ll_element_destroy()
Added implementation for ll_delete()
Added implementation for ll_destroy()
Added implementation for ll_foreach() (macro)
Fix: If the first element gets deleted, we have to re-set the head pointer
Fixup: Remove allocator
Add prequisites for graph inclusion
Ignore metapost output
Add section about operations
Add prelimary section on AVL internal operations
Add subsections about AVL operations