Change to netstandard2.0

[ChaoticIke]

• Changed to netstandard2.0 to support more frameworks.
• Downgraded Microsoft.Extensions.Http to lowest version. Higher frameworks can override this with a newer version.
• Cross target test project (I added netcoreapp2.1 since we haven't updated yet, but it could be removed)

Note:
dotnet frameworks require System.Net.Http from nuget. The <ItemGroup> conditional and net472 target could be moved to Main csproj instead.

Tests:

• Authorization_Tests: Hardcoded data
• Collect_Tests: http://placehold.it/250x250 not found
• Collection_Tests: http://placehold.it/250x250 not found
• CustomerSavedSearchavedSearch_Tests: The following constructor parameters did not have matching fixture data: CustomerSavedSearchavedSearch_Tests_Fixture fixture. Happened when I ran on master branch as well
• Fulfillment_Tests: Hardcoded location
• FulfillmentEvents_Tests: Hardcoded location
• FulfillmentOrder_Tests: Hardcoded location
• RetryExecutionPolicies_Tests: Our test store is shopify plus
• ShopifyException_Tests (Rate exceptions): Our test store is shopify plus

[ChaoticIke]

I've created a sample branch for net4.5 using preprocessor directives, if you still wanted to support earlier than 4.7.2.
It just changes the DefaultHttpClientFactory to not use the interface and removes the method to set a global default

https://github.com/ChaoticIke/ShopifySharp/tree/net45

[nozzlegear]

Thanks for the pull request! I'm planning on reviewing/testing it this week and hopefully merging it if all goes well.

[nozzlegear]

Many days later this has been released in 5.16.0 on nuget! Thanks for the pull request!