Changed the buffer-allocating Container template constructor
* it now multiplies the given size by the size of the structure
** this makes more sense from a user perspective
* added data() member method which returns a pair of the buffer pointer and its size in bytes
** otherwise one can not access the raw container contents to e.g. write them to disk
* updated README.md concerning the dtl::Buffer and Container template changes introduced mainly in a36a6ee3e6f45f118b370c6fc583687419a6b761