Feature: Public Network

Public Network (#47) now is implemented too. 

Contribution team: I would like you to check this out (including documentation grammar). Leave your comments and suggestions.

Quote from updated documentation describes how it works:

### Public Network
It is fully compatible with basic Vagrant [Public Networks](http://docs.vagrantup.com/v2/networking/public_network.html).
#### Available arguments (unnecessary, but provider specific):
- `bridge` - target host's interface for bridged network. You can specify full (ex: `Wi-Fi`) or short (ex: `en0`) name of interface. If omitted, you will be asked to choose the interface during the VM boot (or if only one interface exists, it will be chosen automatically).
- `mac` - MAC address which will be assigned to this network adapter. If omitted, MAC will be automatically generated at the first `up` of VM.
- `ip` - IP address which will be assigned to this network adapter. Use it, if you want to configure adapter manually.
- `netmask` - network mask. Default is `"255.255.255.0"`. It is used only in pair with `ip`
- `type` - IP configuration way, only `dhcp` is available. Use it only if your public network has a valid DHCP server. Otherwise, omit this attribute or use an `ip` and `netmask` described above.
- `nic_type` - type of network adapter. Can be any of `"virtio"`, `"e1000"` or `"rtl"`. Default is `"e1000"`.
_Hint:_ Full names of network interfaces are displayed in _System Preferences -> Network_ window, and short names - in the `ifconfig` command output on your Mac.

#### Example:
```ruby
Vagrant.configure("2") do |config|
  config.vm.network :public_network, bridge: "Wi-Fi", mac: "001C425FC3AB", type: "dhcp"
  config.vm.network :public_network, bridge: "en4", ip: "10.3.1.18", netmask: "255.255.252.0"
end
```
It means that two public network adapters will be configured:
1) The first will be bridged to the 'Wi-Fi' host machine's interface and will have the specified MAC address. After the VM boot it will be automatically configured to get an IP from the DHCP server, which is accessible in the 'Wi-Fi' network).
2) The second adapter will be bridged to the interface 'en4' and will have static ip '10.3.1.18' and mask '255.255.252.0'.