Add methods for each Calibre binary / command

[MrXyfir]

• Methods for each command: calibre.db.*, calibre.ebookConvert, etc
  • calibre.db.add()
    • calibredb add duplicates #8 Among returned data, supply array of duplicate books
• Improve error checking by looking at the command's output and checking for specific values based on the command.
• Return result object that contains the original full output string and also parsed data relevant to the command.
• Each method takes relevant arguments, and not just the general args and options from run().

[MrXyfir]

Added calibre.ebookConvert in v2.1.0.