Support for pointers to primitive types

[ulfurinn]

The only way to have a nullable primitive type in Go is to declare it as a pointer; however, a field like *int makes megajson think it's a nested struct and attempt to use "NewintJSONScanDecoder".

[benbjohnson]

@ulfurinn Thanks for the issue. I have some refactoring to do (hopefully) this week to clean up the types that megajson supports and to do some performance tuning. My goal is to support everything that encoding/json supports.