Get(), Set(), Delete(), DeleteAny() ops on map.

Use jsonpointer to Get, Set, Delete specific fields inside
map[string]interface{}.

- Get(), Set(), Delete(), DeleteAny() are not thread safe.
- Get(), Set(), Delete() are idempotent operations.
- test cases for Set(), Delete(), DeleteAny()

- inplace Incr(), Incrs() and Decr() API.
- use jsonpointer to refer to float64 type field and apply Incr() / Decr() on
  that.
- use jsonpointer to refer to []float64 type field and apply Incrs() on that.
- test cases and benchmark functions.
Benchmark between gojson Vs encoding/json.
optimized parsePointer().
100% coverage.
fixes based on pull-request comments.

- document text.
- Merged Incrs() feature within Incr().
- Added a similar feature for Decr().

- Incr() and Decr() operates on map[string]inteface{}, and value
  can be either []interface{} or []float64{}. There is no way to
  handle them like generics.

refer:
    github.com/dustin/go-jsonpointer/pull/5