Support parse long integer (64bit)

Current request will only parse 64bit integer into float64, which is
default behavious or encoding/json.

The commit will default switch to parse integers into json.Number
instead of float64, let user decide where the integer goes.
- test for get expected value if we have specify the field type correctly
Merge pull request #149 from clsung/support_long_int_parse

Support parse long integer (64bit)