Remove unused sniffer config, add PssdkVersion setting

* Rather than having various places where the PSSDK path is set scattered
  through the solution I created a property which contains the version.
  This means we can update that version property in the project and it'll
  take effect everywhere it's required.
* Removed debug and release configurations from ext_server_sniffer.
Updated build documentation

Added more detail around using the appropriate configurations depending on
whether you have access to the appropriate SDKs.
Build status and documnetation update

* Added the build status notification to the top of the README.
* Updated the order of details in the docs (put Linux build after Windows).
* Updated the details about where this repo came from.

Note: this PR should be merged AFTER #20 for ease of merging.
Land #20, update docs and project files.

Tested by merging and running a build, as proscribed by the README.md.
12 succeeded, 0 dailed, 2 skipped.
Land #22, more doc and proj updates

Again, tested with a successful run of the build.