v0.3.0

Added

• Docs: Add change log and contributing guide.
• Small improvements to the classes and methods docstrings.
• set_text() method for the Text response.
• set_subtitle(), set_image() and set_buttons() methods for the Card response.
• set_title() and set_quick_replies() to the QuickReplies response.

Bug fixes

• Fix Card and QuickReply missing fields.
• Fix optional parameters for all rich responses.
• Fix WebhookClient's parsing of Image and Card responses.
• Fix RichResponse instantiation (shouldn't be able to instantiate an abstract
  base class).

Changed

• Docs: Change theme to Read the Docs' theme.