Rename the cmd binary to be train instead by default

[markbates]

By renaming the github.com/shaoshing/train/cmd package to github.com/shaoshing/train/train the binary will be correctly built as train instead of cmd. This is the more idiomatic way to build go binaries, and probably a lot less confusing for people who end up with a cmd binary in the bins directory like I just did. :)

[jboursiquot]

Any feedback on this and other issues? Would hate to fork this project but it seems that's how peeps get their fixes in.

[logrusorgru]

I'd prefer cmd/train, see #26
A cmd means executable, following common practice.