Feature: Private Networking

I've implemented multi-adapter private networking and fixed #5. Public networking will be implemented near days, it is so easy now.
To other contributors: so, guys, I would like you to check this out (including documentation grammar). Leave your comments and suggestions.

Quote from updated documentation describes how it works: 

## Networking
By default 'vagrant-parallels' uses the basic Vagrant networking approach. By default VM has one adapter assigned to the 'Shared' network in Parallels Desktop.
But you can also add one ore more `:private_network` adapters, as described below: 

### Private Network
It is fully compatible with basic Vagrant [Private Networking](http://docs.vagrantup.com/v2/networking/private_network.html). 
#### Available arguments:
- `type` - IP configuration way: `:static` or `:dhcp`. Default is `:static`. If `:dchp` is set, such interface will get an IP dynamically from default subnet "10.37.129.1/255.255.255.0".
- `ip` - IP address which will be assigned to this network adapter. It is required only if type is `:static`.
- `netmask` - network mask. Default is `"255.255.255.0"`. It is required only if type is `:static`.
- `nic_type` - Unnecessary argument, means the type of network adapter. Can be any of `"virtio"`, `"e1000"` or `"rtl"`. Default is `"e1000"`.

#### Example:
```ruby
Vagrant.configure("2") do |config|
  config.vm.network :private_network, ip: "33.33.33.50", netmask: "255.255.0.0" 
  config.vm.network :private_network, type: :dhcp, nic_type: "rtl"
end
```
It means that two private network adapters will be configured: 
1) The first will have static ip '33.33.33.50' and mask '255.255.0.0'. It will be represented as device `"e1000"` by default (e.g. 'Intel(R) PRO/1000 MT').
2) The second adapter will be configured as `"rtl"` ('Realtek RTL8029AS') and get an IP from internal DHCP server, which is working on the default network "10.37.129.1/255.255.255.0".