v2018.12.10-1 -- Four new examples

Added four new examples:

1. Get the envelope tab data.
   Retrieve the tab (field) values for all of the envelope's recipients.
   Source.
2. Set envelope tab values.
   The example creates an envelope and sets the initial values for its tabs (fields). Some of the tabs
   are set to be read-only, others can be updated by the recipient. The example also stores
   metadata with the envelope.
   Source.
3. Set template tab values.
   The example creates an envelope using a template and sets the initial values for its tabs (fields).
   The example also stores metadata with the envelope.
   Source.
4. Get the envelope custom field data (metadata).
   The example retrieves the custom metadata (custom data fields) stored with the envelope.
   Source.