Create driver model and refactor virtualbox dependencies into driver

[zeeyang]

Laying out some foundation work for multi-hypervisor support. Some keys changes include:

• New Machine interface defined in driver/driver.go.
• Move common data structures from virtualbox into driver package.
• Eliminated all direct references to virtualbox package.
• Added --driver flag. e.g. boot2docker-cli --driver=dummy up
• Added --init flag for optional auto init. e.g. boot2docker-cli --init=true up

[SvenDowideit]

I will look at it in more detail after I get the 1.1.1 b2d release out - very awesome to see though

[SvenDowideit]

can we please not make init implicit? its useful to be able to separate out the creation from the starting

instead, perhaps we should add a --force which does whatever is needed.

[tianon]

I agree with @SvenDowideit that having an explicit init function (even if it's usually implicit) is useful, but all-in-all the refactor looks pretty solid from a visual code review - the interfaces look reasonably idiomatic and I'm excited to see where this goes.

[zeeyang]

Thanks for the feedback guys. I brought back the init command. Auto init is now explicit with the newly added --init=true (default false) flag.

[SvenDowideit]

yes, I like it LGTM - @tianon @gmlewis @crosbymichael @steeve I'd love to merge this and then we can keep working on it :)

[tianon]

LGTM

[gmlewis]

tiny nit: I have been encouraged in peer reviews of my own code to reserve variables like i to be integer index counters for loops... so that others looking at the code are least surprised. So if you agree, you could make this variable mc or m. If you want to leave it as is, that's fine with me too.

[gmlewis]

LGTM after comments addressed.

[zeeyang]

Thanks for the code review. @SvenDowideit I have rebased to lastest master. There are three new commits based on @gmlewis feedback.

[SvenDowideit]

awesome!