Upgrade command

[Tonkpils]

This is a working prototype of the upgrade command for the CLI.

It will install the latest version of the CLI under the same path as the executable that called the command.

One thing that it does not support yet that I'd like to fix before this gets merged:

1. ARM version upgrades
   Golang has no way of checking the ARM version during runtime
   • A possible solution would be to set the ARM version during the build process with -ldflags which would allow us to set the binary's OS/ARCH/ARMV that was used during build.

Also, I wrote the windows portion a bit blindly since I don't have a windows machine to test it on. I'll make sure it works properly when I get a chance.

I'd also like to include a generic message to file an issue here if the install fails with the debug info but not too sure how the message should read. Thoughts?

Once this is merged it fixes #61

[kytrinyx]

This is so cool! I'm at work, so can't read through it now. I think setting the arm version with a build flag is perfect.

[Tonkpils]

Ok, I re-wrote the bin/build-all script to make it set the variables during build. I've tested the upgrade command in linux, windows and mac. Also, if you see any improvements that can be done to the build script let me know.

[kytrinyx]

Yay, doc comments :)

[kytrinyx]

This looks great. I'm going to merge it in (the only thing that I was wondering about is a comment, and that can be tweaked later, I think).