Allow opening multiple endpoints for the same device

This CL fixes the case when you have to open two (or more) endpoints for the same device, interface and configuration. In my case, the device had two endpoints:

```
002.034 1915:7777 Unknown (Nordic Semiconductor ASA)
  Protocol: (Defined at Interface level)
  Config 01:
    --------------
    Interface 00 Setup 00
      Vendor Specific Class (Vendor Specific Subclass)
      Endpoint 1 IN  bulk - unsynchronized data [64 0]
      Endpoint 1 OUT bulk - unsynchronized data [64 0]
    --------------
```
or, alternatively

```
found setup:
usb.InterfaceSetup{
  Number:0x0, Alternate:0x0, IfClass:0xff, IfSubClass:0xff, IfProtocol:0x0,
  Endpoints:[]usb.EndpointInfo{
    usb.EndpointInfo{
      Address:0x81, Attributes:0x2, MaxPacketSize:0x40, MaxIsoPacket:0x0,
      PollInterval:0x6, RefreshRate:0x0, SynchAddress:0x0
    },
    usb.EndpointInfo{
      Address:0x1, Attributes:0x2, MaxPacketSize:0x40, MaxIsoPacket:0x0,
      PollInterval:0x6, RefreshRate:0x0, SynchAddress:0x0
    }
  }
}
```
one endpoint is IN, another is OUT; so, in order to communicate with my quadrocopter, I have to open both endpoints. Since Device.OpenEndpoint calls libusb_set_configuration, it fails on the second time, since the device has already an interface claimed.

This is a standard issue with libusb, and it has a dedicated section on the libusb website: [libusb: Caveats -- Configuration selection and handling](http://libusb.sourceforge.net/api-1.0/caveats.html#configsel).

I hope this patch would be helpful for anyone with two-ways usb devices, but I might be wrong, of course.