Add a strict parsing mode

[davydog187]

The Avro spec is pretty loose in what you're allow to include in a schema. This can be unhelpful when you're trying to write a schema and want the parser to give you detailed information on what is a recognized field versus one that will be ignored.

In #62 I began to add a strict check that any additional fields on a schema would raise an exception. We cannot do this by default as it may cause issues with interop between languages. Instead, we can support this feature by add a strict mode that can be turned on as an option to the parser strict: true.

[davydog187]

See f491098 which removes strict parsing and refers to this commit for future development