Visual Studio 2013 Builds

* Renamed `client/interface` files with `client_` prefix to avoid object conflicts.
* Project file factored into non-redundant properties files for ease of maintenance.
* Fixed a number of x64 numeric inconsistencies (although some remain).
* Cosmetic fixes and fixes for some trivial compiler warnings.
* Ported for Unicode on platforms that are not exclusively Unicode.
* I've [forked libconfig](https://github.com/evoskuil/libconfig) and deployed a [NuGet package](http://www.nuget.org/packages/libconfig/) that the project depends on.
* Other NuGet dependencies are not yet available in `v120` compilation so may fail at run time.
* Until they are updated, other NuGet dependencies require updated targets to link, see:
    * [libssh2-nuget-targets](https://github.com/evoskuil/libssh2-nuget-targets)
    * [openssl-nuget-targets](https://github.com/evoskuil/openssl-nuget-targets)
    * [zlib-nuget-targets](https://github.com/evoskuil/zlib-nuget-targets)