First version of the doubly linked list.

The following features are available in the first version:
- create dll
- traverse dll
- search dll
- create node
- add first node
- add begin
- add end
- add after
- free node
- delete
- delete first
- delete last
- destroy dll
- reverse dll
- sort dll
- print dll
added README.md file
added .gitignore and LICENSE.md

- added .gitignore
- added LICENSE.md
- added GPL header to source files
- cleaned interactive.c
Update README.md
Update README.md
fixed bug in function dll_delete_first and dll_delete_last
ignored compilation "interactive"
bug fixes for dll_search, dll_delete_first and dll_delete_last

bug fixes:
- dll_search -> segfault when searching in empty list
- dll_delete_first -> do nothing if list is empty
 -dll_delete_last -> do nothing if list is empty

- corrected spelling error in the dll_sort doc
added iterator functions in dll.h and dll.c, updated interactive.c

added functions (dll.h and dll.c):
- dll_head
- dll_tail
- dll_curr
- dll_size
- dll_has_next
- dll_next
- dll_has_prev
- dll_prev
added new features to interactive.c
included usage scenarios for iterator functions in the performance
function (interactive .c)
cleaned interactive .c
cleanded dll.c
added .plist files to .gitignore
Update README.md
fixed dll_destroy bug, cleaned up code
added new function dll_add_before, cleaned up code

added new function dll_add_before
added assert statements to the iterator functions
fixed bug in dll_contains function
minor improvements in the code
cleaned up the code
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
updated README.md
added register functions to handle the user defined data structure

removed user defined data structure functions (compareData, freeData,
printData) as function parameters
They are now registered to the list through:
- dll_registerCompareData
- dll_registerFreeData
- dll_registerPrintData
cleaned up code and documentation
updated README.md
removed copyright :-)