Overhauled project structure to enable .NET Standard 2.0 and NuGet, and cleaned-up code

This pull request

• Enables easy publishing of the library to NuGet (also closes #13 (closed) when the NuGet packages are published).

• Makes the library target .NET Framework 4.5 and .NET Standard 2.0 (compilation requires the .NET Core SDK 2.0, which is currently in preview).

• (Breaking change:) Removes support for the .NET Framework 4.0 and PCLs, as both are deprecated.

• Includes XML documentation in the packages (although many public classes are undocumented)

• Removes unused utility code.

• Changes names like GetSync to Get (in some cases, the synchromous methods are removed, they just need a GetAsync().Result to make them synchronous.

• The README displays C# and F# properly.

• The tests are simply run via dotnet test.

• The code was formatted.