Refactor: make major and minor changes to api

Created a module package directory structure to make usage straight
forward. Also added docstrings to improve module usage in scripts.
Fixed typo in main class method and removed deprecated method.

BREAKING CHANGE: removed deprecated method 'download_walpaper'

Author: raycadle

wallhavenapi.py

@@ -0,0 +1,17 @@
+from .wallhavenapi import WallhavenAPI, Category, Purity, Sorting, Order, TopRange, Color, Type
+from .wallhavenapi import RequestsLimitError, ApiKeyError, UnhandledException, NoWallpaperError
+
+__all__ = [
+    "WallhavenAPI",
+    "Category",
+    "Purity",
+    "Sorting",
+    "Order",
+    "TopRange",
+    "Color",
+    "Type",
+    "RequestsLimitError",
+    "ApiKeyError",
+    "UnhandledException",
+    "NoWallpaperError"
+]