Implement the purl spec

[am11]

• Sources are kept close to Java implementation.
• xUnit test framework used
• Test suite data is copied to :/tests/TestAssets/test-suite-data.json, from (https://github.com/raw/package-url/purl-spec/9-use-pkg-scheme/test-suite-data.json)
  • We can improve it to download as a pre-build step, if required
• Travis and AppVeyor CI added (requires admin to sign in to Travis and AppVeyor and add packageurl-dotnet projects to gallery)
• README with usage and MIT license file added

To publish to nuget, admin would require to login to https://nuget.org, create an API key, then execute:

cd packageurl-dotnet

dotnet pack --configuration Release /p:Version=1.0.3          # 1.0.3 is example version

dotnet nuget push packageurl-dotnet/src/bin/Release/PackageUrl.*.nupkg    \
  --source https://api.nuget.org/v3/index.json                            \
  --api-key <API-Key>

Closes #1

[pombredanne]

@am11 Thanks ++ for this. I am not too savvy about C# but I can merge this alright