Change package to com.github.sbt.git

[raboof]

To prevent classpath problems when it is on the same classpath with the sbt-git when it was published with the old

Not for merge yet: need to check whether having 'overlapping' autoplugins confuses sbt.

Refs #216

[raboof]

This indeed breaks: created a project with plugins.sbt like

addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.5.10")

// From https://github.com/sbt/sbt-git/pull/217
addSbtPlugin("com.github.sbt" % "sbt-git" % "1.0.3-M1+6-96835fcb-SNAPSHOT")

Produced:

[error] Some keys were defined with the same name but different types: 'gitReader' (com.github.sbt.git.ReadableGit, com.typesafe.sbt.git.ReadableGit), 'gitRunner' (sbt.Task[com.typesafe.sbt.git.GitRunner], sbt.Task[com.github.sbt.git.GitRunner])
[error] Use 'last' for the full log.