Minor improvements to the ABI

[jasonpaulos]

This PR contains a small set of improvements to the ABI. I've separated them into two categories:

Technical changes

• Remove the appId field in favor of a new networks field on Contract. This change has two benefits:
  • A single contract JSON object can now represent a contract deployed to multiple networks, such as to TestNet and MainNet.
  • A contract JSON object can now describe a contract that has not yet been deployed by omitting the networks field. Previously the appId field was always required.
• If a method has exactly 15 arguments, the last argument is no longer required to be encoded in a tuple.
  • This makes the encoding more efficient for this number of arguments, rather than making the 15th argument appear in a tuple of size 1.
• Clarify that in the case of multiple logged values with the special 4-byte return prefix, the last one is the return value.
• Explicitly allow 0 indexes to be used when passing the transaction sender as an account value or the called application as an application value.

Formatting changes

• Add comments to all JSON definitions and enable code highlighting
• Expand example JSON objects to contain valid fields
• Rename "foreign" types to reference types

[jannotti]

These look like good changes to me.