Make JSONLoader public to support cusotm json loader

[yuichi1004]

I worked on making JSON Loader to be public so that we can implement custom JSON loader.
The topic was already discussed on #99 but not implemented yet.

The changes are little bit bigger than I expected for the following reason.

• I needed to implement JSONLoaderFactory so that Schema uses custom json loader to resolve their ref
• I needed to move schema construction logic to schema.go from jsonLoader.go. Because Schema construction process requires several private func/struct access (like newSchemaPool(), newSchemaReferencePool())

I think this approach is reasonable enough but any opinion?

[petemoore]

I think this PR has bust the string loading.

Simple test case with an empty schema:

package main

import (
    "log"

    "github.com/xeipuuv/gojsonschema"
)

func main() {
    _, err := gojsonschema.NewSchema(gojsonschema.NewStringLoader(`{ "$schema": "http://json-schema.org/draft-04/schema#" }`))
    if err != nil {
        log.Fatal(err)
    }
}

Note, when rolling back this PR, the above code sample executes successfully, as expected. With the PR landed, it fails with:

2016/06/20 20:57:48 Reference {0xc820082900 {[]} false false false false false} must be canonical

Therefore, I think this PR needs to be rolled back. Ideally it would be good to add this as a unit test too. Thanks!

[yuichi1004]

I'm working on bug fixing on the above test case.
It's OK to rolling back the PR meanwhile.

Thanks!

[vbatts]

for cross reference sake: opencontainers/image-spec#162 (comment)