Simple stack data structure added.

Will add header files for this and hash table soon
Added headers for hash table and stack
Changed malloc to calloc in hash table initialiser
modified stack file
Deleted old hash table file

Deleted hash table file with spaces instead of underscores
Updated function names in linked string list to distinguish them from functions in other file.
Now follows the rule that every linked list function has to start with linked(function name)
Updated hash table to use same standard function naming as linked list - with hash(function name) except for tests
Updated Readme
Updated hash and linked list files with new methods.
Created queue structure
Created queue header file
Misc updates