Release samples, move constructors, attenuation

An initial implementation of playing release samples on key up. It seems
a bit messy at the moment. Wrote a move constructor since there's quite
a bit of passing around the gigNote class. Use attenuation so that notes
set their gain properly based on the information in the GIG file,
quite noticeable on some release samples.

Note that to support move constructors I enabled building the plugin
with the C++11 standard.