Make Docker plugin portable so that Windows is supported

[mkotsbak]

Closes #760.

[muuki88]

Thanks for your pull request. Can you fix the compile error and squash the commits?

[error] /home/travis/build/sbt/sbt-native-packager/src/main/scala/com/typesafe/sbt/packager/docker/DockerPlugin.scala:64: not found: value File
[error]   val SeparatorChar = File.separatorChar
[error] 

[mkotsbak]

Ah sorry, forgot that addition. Pushed a new commit now.

[muuki88]

@fiadliel do you have a windows computer at hand to verify it works?

[mkotsbak]

I'll add that I have only tested this on Linux myself (I have troubles running Docker-Virtualbox inside Virtualbox), the error was reported to me by another.

[muuki88]

@mkotsbak no problem :) Most of the contributors use Mac/Linux, so this is only a safety choice to have it confirmed by some systems ( we all know the "it works on my machine ;) ), as the roundtrip time for windows fixes is usually longer.

[fiadliel]

I don't, but I'll try and rope in a friend to try the changes. (Windows has been quite the endangered species in any place I've worked for, for a long time).

[mkotsbak]

Fixed it now.

[muuki88]

I will merge this after the 1.1.0 release.