Simplify creation of passes

[ueman]

We should make creating passes easier, there are a couple of things we could do.

• Adding translations should be easier (maybe building on top of Support for localization in PkPass files #8)
• Provide specific methods for creating each pass type.
  • Example

    PkPass.eventPass(
      // only properties relevant to event passes can be set
    ).write();

    I assume this would make validation also a tad easier
• Automatic resizing and converting to the correct image format for images (could be done based on the image package)