Add async implementation

[meziantou]

• Add a new dependency: Microsoft.Bcl.AsyncInterfaces
• Change the target framework from net45 to net461 as this is the minimum version supported by Microsoft.Bcl.AsyncInterfaces
• Add GitLabCollectionResponse<T>
• Add a unit test to ensure the new async methods follow best practices

fix #45

[jontdelorme]

I like the general approach. It makes it easy to add an async implementation to an interface