changed function names, improved search function, improved iterator functions, cleaned code

changed function names
    "dll_" prefix plus camel case for a descriptive function name
changed
    dll_createNode
    dll_addFirstNode
to static functions
added data type from list->size from unsigned int to unsigend long
iterator functions return NULL if the head or tail is reached, then
they begin from head resp. tail again
behaviour of list->curr improved while searching, adding and deleting
nodes...
added three search modes:
- searchHeadAndTail
- searchTailToHead
- searchHeadToTail
with the new param "mode" the search mode can now be choosen within
dll_search
updated interactive.c
cleaned documentation and code
added sample.c showing some usage examples
added makefile
added test commands file for interactive programm
updated .gitignore