Added SerializerOptions

Integrated patch that ctacke opened on RestSharp project as pull request #404

Adds SerializerOptions and ability to output JSON where null values are ignored.
Just set SkipNullProperties to true and implement your POCOs with nullable
fields for your non-reference value types such as int and bool.

See: http://blog.opennetcf.com/2013/08/22/restsharp-dont-serialize-null-properties/