Added declaration of nestl::list and corresponding tests. Implementation
will be provided soon
Added implementation of list_node and list iterators
Implemented most of list methods. However tests are failed.
Implemented nestl::list and corresponding tests, except merge, splice,
reverse, unique and sort
Initial implementation of list::merge
Added implementation of merge, splice and sort methods.

Also added NESTL_CHECK_LIST_NODE macro for checking contracts of list
nodes