Proposal: Restructure (use build for building)

Hi,

this is just an idea to make `build2` obsolete.

I renamed `scripts` to `tasks`, moved `build/*.go` into `tasks` and updated `CMakeLists.txt` & `.travis.yml`.

Further ideas:
* move `build/*.go` into `tasks/build/*.go`, but I wasn't sure.
* Renaming `python.go` to `gen_python_api.go` would also clearify what it does, just brainstorming ...

IMO it would be nice to be able to work without cmake (CMakeLists.txt is large and hard to read IMO)... therefore it would be nice to have a collection of `go run`'able tasks, e.g. for generating the parsers, getting dependencies, generating `cgo.go`. This changeset would be the first step into this direction.
But cmake would still be available, just running those tasks, because it is so easy to get started with for newcomers.

What do you think?

P.S: I'm going to update the wiki pages if this gets accepted.