Restructure contrib_bots bots to new layout.

In order to make the layout of all bots consistent, this commit
moves each bot into a folder with its name and modifies 'run.py'
so that only bots with such a structure can be executed. 'lib'
gets renamed to 'bots'.

contrib_bots/bots/define_bot/DefineBot/docs.md

@@ -0,0 +1,21 @@
+# DefineBot
+
+* This is a bot that defines a word that the user inputs. Whenever the user
+inputs a message starting with '@define', the bot defines the word
+that follows.
+
+* The definitions are brought to the website using an API. The bot posts the
+definition of the word to the stream from which the user inputs the message.
+If the user inputs a word that does not exist or a word that is incorrect or
+is not in the dictionary, the definition is not displayed.
+
+* For example, if the user says "@define crash", all the meanings of crash
+appear, each in a separate line.
+
+![Correct Word](correct_word.png)
+
+* If the user enters a wrong word, like "@define cresh" or "@define crish",
+then an error message saying no definition is available is displayed.
+
+![Wrong Word](wrong_word.png)
+