Add command-line argument for debugging purposes (-dm) and only print the last_size info when debug is enabled

The first commit on this PR adds a new command line to snabswitch:

```
-dm = Enable developer mode. Enables debug prints and asserts.
```
 * This sets to true a global variable _G.developer_debug that can be
   used on the rest of snabswitch to enable prints, asserts or other
   things that are not intended for production.
    
* Make all other parts of snabswitch where a debug variable was used
  (or should be used) to depend on this one.

The second commit makes the last_size print check to happen only when debug is enabled (previously debug was just an alias to print). Check PR #323 for info about this issue


Maybe we can do something like:
```
   if _G.developer_debug then
      if last_size ~= b.size then debug("size=", b.size) last_size=b.size end
   end
```
?