Validate mixables

This is the patch for https://github.com/jubatus/jubatus/issues/627. 

In this patch, Jubatus alert with unsupported mix strategy combination on boot time.

```
$ jubaclassifier -n test -z localhost:2181 -x skip_mixer
F0421 11:49:52.136503 17619 server_util.hpp:144] Dynamic exception type: jubatus::server::framework::mixer::unsupported_mixables::what: skip_mixer does not support this configuration
```