Register class, BulkCreate/Delete, Flasgger, and other improvements

[tschaume]

This PR is a fresh start of #124. It includes:

• A separate register_class function to use with Flask Blueprints (see e.g. here). This should supersede #85 and #115.
• The register_class function also defines endpoints for each URL to enable integration with Flasgger (see this commit).
• Implementation of BulkCreate and BulkDelete methods.
• A typ property for operators to enable easier integration with Swagger definitions (see here).
• Automatically trying to convert values to floats for numeric operators.
• Static get_optional_fields method to allow retrieving optional fields from the class (see here)
• Ensuring that order is maintained in get_requested_fields().
• Forwarding the view_method when serializing.
• Support for Decimal128 in MongoEncoder.
• Improved error handling and kwargs forwarding.
• Bugfix for #129.
• mimerender dependency updated to include martinblech/mimerender#36

@wojcikstefan @thomasst I'm working on getting the tests to pass and implement new ones for BulkCreate/Delete. Would you mind starting the review of this PR in the meantime? Thanks! I will base new PRs for additional functionality currently included in #124 off this PR.

[tschaume]

For anyone interested in this PR, I started maintaining this (and more) through the flask-mongorest-mpcontribs package.