Fix ArgumentNullException for InMemoryRulesStorage from IServiceCollection

[luispfgarces]

Description

• Fixes extension method for in-memory rules data source using IServiceCollection.
• Fixes all integration tests to use in-memory rules data source provider instead of a simpler implementation embedded in tests (which was removed).
• Changes the namespaces of the extension methods for configuring the in-memory rules data source (for simplicity of the consumers of Rules.Framework).
• Refactors unit tests for the in-memory data source provider, by moving them to the Rules.Framework.Tests project. Also, converted the touched files to use var as per convention.

The integration tests for the in-memory data source provider are also candidates for moving to project Rules.Framework.IntegrationTests, but I intentionally left that for another time.

Closes #141.

Change checklist

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• I have covered new/changed code with new tests and/or adjusted existent ones
• I have made changes necessary to update the documentation accordingly

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[luispfgarces]

@carlosgoias @jdromeiro can you please review it again? Unfortunately, I had to resolve merge conflicts from master.

[luispfgarces]

@jdromeiro can you please review this again?