Add to_json Deprecation Warnings

Addresses the first half of #267

Add deprecation warnings on `Bitcoin::Protocol::Tx#to_json(_file)` as well as
to `Bitcoin::Protocol::Block#to_json(_file)`.

This causes the tests to display a lot of warnings and the tests and examples
using these methods must also be removed / revised once they are gone for good.

lib/bitcoin/protocol/block.rb

@@ -228,15 +228,15 @@ def bip34_block_height(height = nil)
         nil
       end
 
-      # convert to json representation as seen in the block explorer.
-      # (see also #from_json)
+      # <b>DEPRECATED:</b> This method will removed in a future release.
       def to_json(options = { space: '' }, *_)
+        warn "[DEPRECATION] Bitcoin::Protocol::Block#to_json is deprecated and will be removed in a future release."
         JSON.pretty_generate(to_hash(options), options)
       end
 
-      # write json representation to a file
-      # (see also #to_json)
+      # <b>DEPRECATED:</b> This method will removed in a future release.
       def to_json_file(path)
+        warn "[DEPRECATION] Bitcoin::Protocol::Block#to_json_file is deprecated and will be removed in a future release."
         File.open(path, 'wb') { |f| f.print to_json; }
       end
 

lib/bitcoin/protocol/tx.rb

@@ -451,14 +451,15 @@ def to_hash(options = {})
         h
       end
 
-      # generates rawblock json as seen in the block explorer.
+      # <b>DEPRECATED:</b> This method will removed in a future release.
       def to_json(options = { space: '' }, *_a)
+        warn "[DEPRECATION] Bitcoin::Protocol::Tx#to_json is deprecated and will be removed in a future release."
         JSON.pretty_generate(to_hash(options), options)
       end
 
-      # write json representation to a file
-      # (see also #to_json)
+      # <b>DEPRECATED:</b> This method will removed in a future release.
       def to_json_file(path)
+        warn "[DEPRECATION] Bitcoin::Protocol::Tx#to_json_file is deprecated and will be removed in a future release."
         File.open(path, 'wb') { |f| f.print to_json; }
       end