Add generic enum mapper

[DSteve595]

This mapper simply uses the enum's name to serialize and deserialize any enum class without having to write a converter.

Tested with:

@JsonObject
public enum TestEnum {
    First, Second, Third
}

Serializing a TestEnum to JSON seems to work, but I can't be 100% sure as I can't figure out how to use this project's test framework. It did spit out proper-looking JSON when I tested it by just serializing a list of them, but I haven't been able to get the annotation processor working in my project, so I currently can't test with any custom classes.

Deserializing this object:

{
  "the_enum": "First",
  "another_enum": "Third"
}

with LoganSquare.parseMap() yields this:

which looks good!

I mainly wanted to do this because after switching my project over from Gson, I missed the ease of how it handled Enums. This still needs work but I think it's a good starting point. Hopefully someone better versed in annotation processing can figure out a clean way to have field names overridden (like @SerializedName on an enum value in Gson).

Thanks for the great library!