Move JSON functions under the Bugsnag namespace

Move monkey-patching of httparty to its own file.
JSON: use more consistent method name with MultiJson
MetaDataHash: rename #meta_data to #to_h

Because it really returns a hash.
spec/json_spec.rb: add some basic tests

Since we add our own layer for JSON, it would be wise to write a couple
of simple tests.
Dump code (R.I.P.)