Global release switch, tag prefix

Let the user decide whether the release should be global or local by setting the globalRelease setting.

Let's say we have a project with the following modules:
- core
- lib-A
- lib-B
- client
- server

You might want to release core, lib-A and lib-B at once but client and server independently.
Currently the release plugin set the version to the version.sbt file at the root of the project, in the ThisBuild scope. Therefore your only option is to create 3 independent SBT projects.

With these changes you can have a per-module release process.

For instance in server the release process could be defined this way:

```scala
Project("server").
      settings(
          ReleasePlugin.releaseSettings : _*).
      settings(
          globalRelease := false,
          tagPrefix := "server-",

          // server specific settings used by the bundle, upload and restartServer steps
          bundleTarget <<= target(_ / "dist"),
          bundleFile <<= (bundleTarget,name,version){(t,n,v) => t / (n + "-" + v + ".tar")},
          serverUri := "john@my.server.com",
          serverTarget := "~/server/versions/",

          releaseProcess := Seq[ReleaseStep](
            checkSnapshotDependencies,
            inquireVersions,
            runTest,
            setReleaseVersion,
            commitReleaseVersion,
            tagRelease,

            // publishArtifacts, // don't care

            bundle,
            upload,
            restartServer,

            setNextVersion,
            commitNextVersion,
            pushChanges
          )).
```