Support application specific error handler/logger

Request

when I integrate jsPDF in another application, it should be possible to connect it with the application's error message / logging approach.

Proposed solution

1. Logging is handled by a logger object

2. logger object is a read/write property of the jspdf instance

3. there is a default logger object which writes to the console

4. the logger object has the methods message, error, warning, info, debug. Maybe we implement the api defined as http://log4javascript.org/

   • warning is used for situation where jspdf can work around (e.g. by proper defaults) deprecation messages would be warnings
   • error is used for serious issues such as wrong parameters etc.
   • info may be used for informational message (e.d. to expose the jspdf version) - probably not applicable for jspdf
   • debug is used to write internal information for debugging purposes
   • message is used to give messages to the user - probably not applicable for jspdf

Start by locating jsPDF's current error, warning, and console logging paths; no files or tests are identified in the issue. Define the logger object's message, error, warning, info, and debug methods, its default console-backed behavior, and the read/write instance property, then verify existing logging still works through the selected logger.