For #137: Set the stage to pack and push 31.0.0-pre1 CefSharp NuGets 

- Bump `CefSharp.*` version series from `3.<something>` to the new `31.0.0-pre1`. Note with this we don't even have a clash with unofficial version id's pushed to staging.nuget.org :smile: 
- Change `cef.redist` dependency version indicator to allow auto-update or install of latest & greatest in the 3.1650.* series (this == less maintenance of dependencies across repos and we should be safe because we are in control of what gets pushed from the cef-binary repo)

----

**Note:** If you don't want to wait for the official NuGets to appear at  www.nuget.org/packages/CefSharp.Wpf  - but wan't some CefSharp 31.0.0 *now* - you can easily roll you own and depend on them by doing this:

0. Build CefSharp from sources
1. Modify the version in `pack.bat` to your own version e.g. `31.0.0-homegrown-pre1` to avoid confusion later on.
2. Run `pack.bat`. For now ignore the error regarding the missing WinForms NuGet
3. Put the `.nupkg` files in a local folder somewhere and set it up as a package source in VStudio

*Just be aware of one thing if you do this;* the binaries you build will not have a dependency on the Visual C++ 2012 Redist (as mentioned in the FAQ) but the Redist matching the VS201x version you build with!