* Progress on VS 2017 build tooling
* Finally, a working VS 2017 tooling build

Due to the way VS test works (by injecting an executable entry point), the performance tests needed to be factored out. I also organized all of our existing tests better along the way.
Actual code changes to Dapper itself are very minor, only formatting and documentation fixes (which we need many more of).

The build.ps1 script is upated to work, but note that <frameworkAssemblies> is not working in .nuspec inside the packages in the new .csproj system. I consider this to be a blocker. Issue is here: https://github.com/NuGet/Home/issues/4853

Related: #683

Note: build.sh needs love from OS X/*nix later to match

This fixes null refs with the DNX MSBuild targets when building from
Visual Studio.

Note: on a nuspec comparison, the following dependencies aren't there
anymore: Microsoft.CSharp, mscorlib, System, and System.Core aren't
explicitly specified in frameworkAssemblies anymore. The only one of
these I can imagine mattering is Microsoft.CSharp.

.Net Core: remove floating versions (use stable ones on Nuget.org) and get this puppy back in action.

Also: it's 2016.

Heh, AspNet and DNX (only) work in VS2015; DNX and DotNet (only) work in VS15; I guess we need to use DNX for now, then, as the only one that works in both.

Recreate all the xproj to reflect the current tooling - old versions break VS15; enable XDocument etc in corefx (#if block was too large)

This eliminates the dual solutions and brings Contrib into the
project.json fold.
- Tests are ported to xUnit though SQLite is disabled on DNX due to
Issue #375 (Interop loads fail in the VS runner and  params are busted
regardless).
- Projects are consolidated down to what's currently possible with
project.json.
- Contrib moved to: Dapper.Contrib and Dapper.Tests.Contrib (so tests
appear at the end, no other reason vs. Dapper.Contrib.Tests)
- EF moved to: Dapper.EntityFramework(.StrongName)
- Rainbow moved to: Dapper.Rainbow
- SqlBuilder moved to: Dapper.SqlBuilder
- Packages are removed since it's not current-tooling friendly to keep
them (nor was it using them).
- xUnit implemented in Contrib via abstracts - this allow us to very
quickly add any database provider to the rig in a very concise way.
- Contrib unit tests fixed in various places - they were not
parallel-runner friendly
- Nuspecs removed, since they aren't used anymore (to eliminate
confusion)
- Removed speed tests for libraries that aren't on current versions of
.Net (what would we be comparing?)
- Perf tests moved to Dapper.Tests (consolidation from
Dapper.DNX.Tests), but aren't activated yet (no define)