db/AuthorManager.js: renamed doesAuthorExists() -> doesAuthorExist()

Removed the 's' for consistency with the other `doesFooExist()` manager calls.
Retained an alias for plugins that might be using it.

src/node/db/AuthorManager.js

@@ -40,7 +40,7 @@ exports.getColorPalette = function() {
 /**
  * Checks if the author exists
  */
-exports.doesAuthorExists = thenify(function(authorID, callback)
+exports.doesAuthorExist = thenify(function(authorID, callback)
 {
   // check if the database entry of this author exists
   db.get("globalAuthor:" + authorID, function(err, author) {
@@ -50,6 +50,9 @@ exports.doesAuthorExists = thenify(function(authorID, callback)
   });
 });
 
+/* exported for backwards compatibility */
+exports.doesAuthorExists = exports.doesAuthorExist;
+
 /**
  * Returns the AuthorID for a token.
  * @param {String} token The token