Network interfaces

Moving the netifaces code from the old pcaprub lib into its own gem.

 - [x] install the new gem (bundle install)
 - [x] launch ```rvmsudo msfconsole```
 - [x] ```use auxiliary/scanner/discovery/arp_sweep```
 - [x] set ```RHOSTS``` to something on your network
 - [x] ```run```
 - [x] you should not see any errors or the following message
```[-] WARNING : Pcaprub is not uptodate, some functionality will not be available```
```[-] Auxiliary failed: RuntimeError SHOST should be defined ```
```[-] Call stack:```
```[-]   /home/todb/git/rapid7/metasploit-framework/modules/auxiliary/scanner/discovery/arp_sweep.rb:54:in 'run_batch' ```
```[-]   /home/todb/git/rapid7/metasploit-framework/lib/msf/core/auxiliary/scanner.rb:164:in `block in run'```
```[-]   /home/todb/git/rapid7/metasploit-framework/lib/msf/core/thread_manager.rb:100:in `call'```
```[-]   /home/todb/git/rapid7/metasploit-framework/lib/msf/core/thread_manager.rb:100:in `block in spawn'```
 - [x] arp_sweep should discover some hosts
 - [x] manually set `INTERFACE` to a non-default interface (eg, `wlan0`).
 - [x] set another sensible target range for `RHOSTS`
 - [x] run again, see responses and no errors.
 - [ ] consult #8023, #7943 for more information