Re-organized tree to prepare for version 2.0.0.

[redboltz]

See https://github.com/msgpack/msgpack-c/wiki/v1_1_cpp_versioning

[redboltz]

This is a pretty big modification to prepare version 2.0.0.

I added the following sub directories:

include/msgpack/v1
include/msgpack/v2

Current API version is v1. The files located on include/msgpack/ are moved to include/msgpack/v1.

For example:

Before moved:

include/msgpack/pack.hpp # v1 implementaion

After moved:

include/msgpack/pack_decl.hpp # include v1/pack_decl.hpp and v2/pack_decl.hpp
include/msgpack/pack.hpp # include pack_decl.hpp and v1/pack.hpp.
include/msgpack/v1/pack_decl.hpp # v1 minimal declaration
include/msgpack/v1/pack.hpp # v1 implementaion
include/msgpack/v2/pack_decl.hpp # v2 minimal declaration. By default, using v1::*

MSGPACK_DEFAULT_API_VERSION is changed to 2 from 1. There is no v2 implementation for now, so the default behavior should be the same as v1 even though MSGPACK_DEFAULT_API_VERSION is 2. I updated the travis-ci, it includes multiple API version versions tests.
@jpetso, @davidchappelle, I have recognized you are great reviewer. Could you check the PR and test on your projects?

Users can set MSGPACK_DEFAULT_API_VERSION by defining the macro as follows. g++ -DMSGPACK_DEFAULT_API_VERSION=1 ...

When we update the code, we need to update an appropriate code.

If the update is a breaking change, update the code located on v2 directory. For example,

include/msgpack/pack.hpp # include pack_decl.hpp, v1/pack.hpp, and v2/pack.hpp.
include/msgpack/v2/pack.hpp # add the breaking change here

Otherwise, update the code located on v1 directory.

Note: Adding new APIs is NOT a breaking change.

[redboltz]

There is no problem reported so far. I will merge the PR tomorrow.