Support external statemanager in VM constructor

[axic]

Fixes #33.

[axic]

@kumavis do you think this is enough to solve #33?

@yann300 this should also simplify Remix's code.

[holgerd77]

Probably will already ease things. I have a bit the fear, that the API remains too implicit, if StateManager is still coupled with the VM and not documented explicitly. Would it still be worth the work/pursue the idea to publish this as a separate module like proposed by @wanderer in the issue #33 description?

Just started to wrap my head around this though, so have not formed an opinion myself on this.

[axic]

The goal of this is to make it less error prone for those who already modify it to do so. State manager should be cleaned up and/or redesigned but that may take a while (forever?). Probably a good middle ground is having this patch, but marking opts.stateManager as EXPERIMENTAL so that we can make breaking changes to the statemanager API as we do not commit to the current one.

[jwasinger]

Keeps backwards compatibility while offering additional flexibility to ethereumjs-vm users. I approve!

[axic]

@holgerd77 added a bold EXPERIMENTAL warning to it.

@jwasinger please reapprove if still happy

[holgerd77]

Like this EXPERIMENTAL placing in the API docs, will approve.