Added support for the field name type.

Since type is a keyword in Julia we need to make sure that if the protobuf
message has a field name 'type' that we do not use that for the Julia's type
attribute name. Instead we'll replace it with '_type'.

This same solution or something similar should likely be employed for other
Julia keywords.
removed unnecessary gen definition
Merge pull request #15 from analyzere/remove-unnecessary-gen-definition

Removed unnecessary function gen(instream, outstream, errstream) definition
Merge pull request #17 from analyzere/support-the-field-name-type-pr

Added support for the field name type.
Added support of messages without fields.

This will support messages which just have enums or nested messages or messages
that are completely empty.
Changed the try/catch to an if/end.

Changed the try/catch introduced to support messages without fields to an if/end
block as per tanmaykm's suggestion.
Added test messages for messages without fields