Throw std::invalid_argument instead of using assert
Add memory allocation functions to Methcla_Host plugin interface

Closes #71.
Use methcla's memory allocation functions instead of malloc/free
Use std::unique_ptr for exception safe memory allocation
Rename SoundFileHandle::Destruct to Destructor
Include cstdio for SEEK_SET
Remove unused variable
Build libmethcla on PNaCl with libmpg123 support