Enable customizing extension libraries (#19).

Support editing `ext/lib`.

Usage:
```
asakusafwOrganizer {
  // common settings for all profiles
  extension {
    libraries += ['<dependency-notation>', ...]
  }
  profile.<name> {
    // profile specific settings (inherits common settings)
    extension {
      libraries += ['<dependency-notation>', ...]
    }
  }
}
```
Merge remote-tracking branch 'private/issue/19' into 0.7-develop
Fixed #20.

Custom assemblies should follow its dependencies.

* 'assemble' task with following script generates a distribution archive with '/batchapps-sources'
```
asakusafwOrganizer {
    asakusafwVersion asakusafw.asakusafwVersion
    profiles.prod {
        assembly.into('batchapps-sources') {
            put { sourcesJar }
        }
    }
}
...
task sourcesJar(type: Jar) {
    classifier 'sources'
    from project.sourceSets.main.allJava
}
```
Refactoring gradle plugin (#21).

Explicitly create `.settings` dir for eclipse.
Merge remote-tracking branch 'private/issue/20' into 0.7-develop
Merge remote-tracking branch 'private/issue/21' into 0.7-develop
Use project.apply(plugin: ~) instead (#22).

In Gradle 2.3, `project.plugins.apply()` method will be deprecated.
Add `updateAsakusafw` task to organizer (#23).
Merge remote-tracking branch 'private/issue/22' into 0.7-develop
Merge remote-tracking branch 'private/issue/23' into 0.7-develop
Merge branch '0.7-develop'