support binary version in launcher

See [support for _2.10 suffix by default in launchconfig](https://groups.google.com/d/topic/simple-build-tool/Wsyjmj_ASU8/discussion).

### steps

Save the following as `/Users/foo/bar/launchconfig`.

```
[app]
  version: 1.0.1
  org: org.scalaxb
  name: scalaxb
  class: scalaxb.compiler.SbtApp
[scala]
  version: 2.10.0
[repositories]
  sonatype-public: https://oss.sonatype.org/content/repositories/public/
  maven-central
```

Then run:

```
$ java -Xmx1G -jar ~/Applications/sbt/sbt-launch-0.12.0.jar @"/Users/foo/bar/launchconfig"
```

### problem

```
Getting org.scalaxb scalaxb_2.10.0 1.0.1 ...

:: problems summary ::
:::: WARNINGS
		module not found: org.scalaxb#scalaxb_2.10.0;1.0.1
```

### what this changes

1. `cross-versioned` property is expanded to `true|false|none|binary|full`.
2. `cross-versioned` now defaults to `binary`.
3. when `cross-versioned` is `binary` it uses `LaunchCrossVersion.binaryScalaVersion(sv)` to build the _2.10 style suffix.