Port net-ipfs-api to .net core

[lvermeulen]

Unit tests were changed to XUnit.

[jeremy-ellis-tech]

Much appreciated changes. Especially around the C# 6.0 features.

However, some changes move away from well established code conventions.

For example:

• Implicitly typed local variables where it is not clear what the type is from either side of an assignment.
  ie. HttpContent content = await ExecuteGetAsync(...); vs var content = await ExecuteGetAsync(...);
• static/const member in ALL_CAPS_SNAKE_CASE instead of PascalCase
• Using primitive alias instead of class name for accessing static members. ie. string.Equals(..) vs String.Equals(..).

[lvermeulen]

Ah. I follow CoreFX guidelines myself - https://github.com/dotnet/corefx/blob/master/Documentation/coding-guidelines/coding-style.md - but I recognize I may have transgressed even those. I will fix and resubmit.

[timothyparez]

If we're going to have .NET core support, shouldn't it be in a separate branch?
If we want to keep everything in one branch we should target .NET Standard instead.

[lvermeulen]

@timothyparez That's the general idea.