Added target for .NET Standard 2.0

Add target for .NET Standard 2.0 (#23)

* [NoTicket] - Added support for .NET Standard 2.0, DotNet Core 2.0 and DotNet Core 2.1.

* [NoTicket] - Removing some build warnings.
* Removed explicit targetting for DotNetCore; just targetting Net Standard now.
* Upgraded test project to .NET 4.7.1
* Upgraded test project dependencies.

* [NoTicket] - Attempting to fix the build caused by poorly formatted appveyor.yml.

* [NoTicket] - Attempting to fix an issue with the appveyor.yml.

* [NoTicket] - Attempting to fix the build by adding a NuGet restore step.

* [NoTicket] - Attempting to fix the NuGet packaging phase of the build.

* [NoTicket] - Modifying the build to append a pre-release suffix for non-master builds.

* [NoTicket] - Fixing an issue with the appveyor.yml build configuration.

* [NoTicket] - Attempting to fix a broken build due to missing carriage return.

* [NoTicket] - Attempting to fix an issue with the code coverage phase of the build.
The wrong version of the NUnit Console Runner was being referenced.

Added async support

#18

Added support for PATCH

v1.3.124

[NoTicket] - Bumping the version minor number, as the API contract ha…

Added multi-target support, including for .NET Core

v1.1.111

[NoTicket] - Bumped the minor version number, now that the library su…

Added support for JSON attributes

[https://github.com//issues/7] - Added support for JSON attributes

Support for NETStandard1.1

v1.0.69

[NoTicket] - Removed the build cache from appveyor.yml, as it doesn't…

Support for .NET 4.5

[https://github.com//issues/5] - Added support for .NET 4.5.

Support nullable properties

Updated the deseriliser to support nullable properties.

Allow custom implementations of IJsonHttpClient

v1.0.39

[https://github.com/eoin55/HoneyBear.HalClient/issues/3] - Added two …

Allow overriding of MediaTypeFormatters in HalClient

v1.0.34

[https://github.com/eoin55/HoneyBear.HalClient/issues/1] - Defaulting…