Deprecate and then remove to_json methods

[etscrivner]

The to_json methods on blocks and transactions are somewhat out of place. Originally they were used to support a blockchain explorer, but this use-case no longer exists. The place to phase these out would take the following steps:

1. Add a deprecation warning to let users know these methods are going away and do a release.
2. Completely remove the methods and all associated tests in a follow-on major release.

[etscrivner]

@lian It sounds like from your comment on #280 that you'd want to deprecate and remove all JSON methods, meaning to_json* and from_json*. Is this accurate?

[lian]

yea, think we should deprecate them and remove move all of them. unsure about to_hash and from_hash, but even if we keep them we might remove their script.to_string output and just output scripts as hex

[etscrivner]

@lian Updated, let me know if this looks good.