add mouse support

[elringus]

Resolves #19.

[eschewary]

Ok, thank you for your work. I actually need mouse buttons 4 and 5 to work too, I'm sorry I should have specified it. I did some digging after integrating your additions. These buttons are included in the RAWMOUSE structure documentation mentioned in my issue #19 as the 'RI_MOUSE_BUTTON_4' and 'RI_MOUSE_BUTTON_5'. They're defined, I guess together, as 0x020B (Down) and 0x020C (Up).

I added them into RawMouseState.cs:

but it's not functioning. Is there a way to add these buttons?

Also is there a way to use that whole 'typedef struct tagRAWMOUSE' from here? It looks like it already has separate definitions for XBUTTON1/XBUTTON2 down/up:

[elringus]

Here are the supported events: https://www.pinvoke.net/default.aspx/Constants/WM.html Both xbutton 1 and 2 are unfortunately mapped to a single event, so I'm not sure if it's possible to sample them individually. I've pushed 1.3 release with xbutton support.