Add JsonIgnore Attribute

I've added an attribute which can be applied to fields and properties to ignore them during JSON serialization and deserialization.  Using `[JsonIgnore(JsonIgnoreWhen.Serializing)]` will not serialize the field but still deserialize data into it.  `[JsonIgnore(JsonIgnoreWhen.Deserializing)]` will serialize the field but not deserialize it.  The default `[JsonIgnore]` ignores the field during serialization and deserialization.

I've also added some initial support for importing/exporting floats and signed long ints.  I did this because LitJson works very well with Unity which uses floats for most floating-point numbers.

Unit tests also included.