Clean up Visual Studio build.

* Make the `playd.vcxproj` file use relative paths to include and lib
directories.
* Ignore these directories in `.gitignore`.
* Move the `libsox.def` example out into its own file.
* Edit the Visual Studio build notes to reflect the above changes.
Merge pull request #45 from UniversityRadioYork/mw-vs-build

Clean up Visual Studio build.