Add support for nonstandard double argument type:

 'd' => 64 bit ("double") IEEE 754 floating point number

See http://opensoundcontrol.org/spec-1_0 for a list of the nonstandard types.

This type is used by the SuperCollider server.
Merge branch 'add-double' of https://github.com/samaaron/osc-ruby into samaaron-add-double
adding tests for double64 type