Remove underscore dependency

Currently angular-kendo code uses underscore utility functions to get list of registered kendo widgets through jQuery.fn. While underscore is very popular and widely used library, this puts a hard dependency on underscore and it becomes mandatory for applications to include it even if they are not using it.

The underscore utility functions can be easily replaced by a simple for-in loop to iterate over jQuery.fn properties and create the widget list. This pull request contains corresponding code.