Allow extending Meta with user-defined behaviour

[aflatter]

Hey,

My goal was to override encode and decode in order to allow special processing for specific buckets.
A real world example would be something like:

decode = (data) ->
  json = JSON.parse data
  json.id = @key
  json
encode = (data) ->
  delete data.id
  JSON.stringify data

Unfortunately I currently can not provide custom Meta objects currently as they are passed to the Meta constructor anyway. What I do at the moment looks like:

Meta = require 'riak-js/lib/meta'
Meta::encode = (data) ->
  super unless @bucket == 'foo'
  # do something

A nice way to do that would be:

class FooMeta extends Meta
  decode = (data) ->
    # ...

meta = new FooMeta()
db.save 'foo', {hello: 'world'}, meta, (err, res) ->
  # hi

What do you think? :)

[frank06]

I think it's a good idea but I need to closely look into it, and rework some parts.

[frank06]

See https://github.com/frank06/riak-js/commit/303ddffc4a900ad334bbbfd3cb3608aea28e6353#L1R224