Go SWIG support

This commit adds a Go SWIG wrapper, and adds the autoconf option "--with-go" to build libotapi-go.so.

For a Go package to use this wrapper, it would use this import line
```go
include "github.com/FellowTraveler/Open-Transactions/swig/glue/go"
```

The wrapper is a total mess from the Go perspective, so a reasonable package would import otapi then wrap the SWIG code into idiomatic Go. That means the end user would select a third-party Go package that imports otapi internally without worrying about the aforementioned import line, and would simply have to ensure that libotapi-go.so was present.