Moved duplicated codes in “tbl_addfield.php” and “tbl_create.php” to single a file "create_addfield.lib.php".
Refactored the code using “Extract method” refactoring technique.
Updated the description of the "create_addfield.lib.php" file
Renamed the function names, using "column" instead of "field".
Corrected indentation according to cording standards.
Fixed typos.
Merge pull request #348 from hisamith/refactor_tbl_create_and_add_field

Refactored “tbl_addfield.php” and “tbl_create.php” by moving duplicated code