Fix case sensitive filenames in csproj files

Fixes build on GNU Linux/Mono platform:

    juergen@samson:~/tmp/octokit.net/Octokit → xbuild Octokit-Mono.csproj 

    : error CS2001: Source file `Clients/IOauthClient.cs' could not be found
    : error CS2001: Source file `Clients/OauthClient.cs' could not be found