Added annotation to be able to ignore fields when creating schema from @Tag

Hi,

I'm not sure if you want this merged but I needed it for my project so I thought that it was best to throw you a Pull Request. Feel free to merge if you like it.

I created an annotation ProtoIgnore that can be used on fields to be able to ignore them in the schema creation. Do tell me if you do not like the name 'ProtoIgnore'.

Therefore you can now have a class that looks like:

class A {
   @Tag(1)
   int field1;
   @Tag(2)
   String field2;
   @ProtoIgnore
   double field3;
}

I wrote unit tests for this and I've tested it in my own project.

Best,
Johannes