Re-normalize repository

According to .gitattributes Docs[1].

[1]: http://git-scm.com/docs/gitattributes#_end-of-line_conversion
Merge pull request #69 from alextercete/feature/repo-re-normalization

Re-normalize repository.

Thanks @alextercete!
Avoid NullReferenceException with async methods

When using async methods and loose Mock Behavior, calls to Task methods
give NullReferenceException, since default(Task) is null. Calls to
Task.Wait() and Task<T>.Result should follow the loose behavior: do
nothing and return the default value for T, respectively.

Related to #64.
Make code .NET 3.5 compatible

Use compile directives to ensure smooth NuGet package generation.

'IReturnsExtensions' was using 'System.Threading.Tasks' namespace, so
compile directives were also added to it.