Feature/matchers/expect json sizes

[PikachuEXE]

Besides the README & code & spec, I have:

• Add CHANGELOG
• Make expect_json_types accept Proc as expectation (not expect_json though)
• Rename some variables & add some blank lines for readability

[coveralls]

Coverage increased (+0.02%) when pulling 4fb810a on PikachuEXE:feature/matchers/expect-json-sizes into e453629 on brooklynDev:master.

[jsvisa]

I think @PikachuEXE you should update the gem version the same time, because it add some new apis.:)

[PikachuEXE]

I think the versioning is the author's responsibility

[jsvisa]

@brooklynDev , It's better to add the gem version along with the release