DRAFT: Add new rule C#-FriendlyAsyncOverload

[parhamsaremi]

Fixes #517

NOTE: this PR supersedes an older PR

[knocte]

This PR needs to change because we're going to change the convention: instead of having Foo(): Async<'T> for the F# method and FooAsync(): Task<'T> for the C# one, we're gonna have AsyncFoo(): Async<'T>, FooAsync(): Task<'T>, and Foo(): 'T for the synchronous version. This conclusion comes from a debate between me and Afshin in this PR: nblockchain/NOnion#56

[Thorium]

Not all the libraries are meant for C# use. For example type-providers, they just don't work in C#.

[knocte]

Not all the libraries are meant for C# use. For example type-providers, they just don't work in C#.

Good point, that's why this rule won't be enabled by default.