Extend FsPickler static methods to accept IPicklerResolver parameter.

[mchylek]

This PR tries to resolve issue I found as part of extending Vagabond to use custom IPicklerResolver. I've noticed it works fine with Serialization but Vagabond.ComputeObjectDependencies use FsPickler.VisitObject that's currently missing support for providing custom resolver.

This change is not binary compatible as it's add optional parameter but I'm happy to update it as new methods overloads if needed.

It's partially related to #123

[mchylek]

@eiriktsarpalis could you take a look on this PR? 😄

[mchylek]

@eiriktsarpalis could you take a look on latest changes? Thanks!

[eiriktsarpalis]

I'm pretty swamped with regular work at the moment, I'll take a look at some later point. Sorry!