File getBooksByTitle.php added.
It returns all the information of a book with a previous chosen title
File getBooksByCategory.php updated.
Added comments to an easier understanding of the code
File getCategories.php updated.
Added comments to an easier understanding of the code
File getFirstBooks.php added.
It returns the first N books of each publisher
File getNewBooks.php added.
It returns the new books or the older ones, according to the user's preference