Fixed #367.

* Keeps batchapps in ASAKUSA_HOME clean when running TestDriver.
* Introduce ASAKUSA_BATCHAPPS_HOME.
Merge remote-tracking branch 'private/issue/367' into 0.6-develop
Fixed #368.

Improves details of Batch Application Plugin [Gradle Plugin]
* Introduce convention mappings to Batch Application Plugin tasks.
* Intorduce SourceDirectorySet for DMDL.
Merge remote-tracking branch 'private/issue/368' into 0.6-develop
Fixed #369.

Gradle Plugin supports ThunderGate.
Merge remote-tracking branch 'private/issue/369' into 0.6-develop
Refactor SourceSet facilities (#368, #369).
Merge remote-tracking branch 'private/issue/368-M1' into 0.6-develop
Fixed bugs duplicating annotation dir (#368).
Merge remote-tracking branch 'private/issue/368-M1' into 0.6-develop
Add tests and refactor for Gradle Plugins (#368).
Merge remote-tracking branch 'private/issue/368-M1' into 0.6-develop
Revise Gradle plug-in build settings (#368).
Introduce 'asakusafw.basePackage' (#368).

Changed 'asakusafw.modelgen.modelgenSourcePackage' and
'asakusafw.compiler.compiledSourcePackage' to refer the
'asakusafw.basePackage' as their base package name.

Build will be failed if neither 'asakusafw.basePackage'
nor 'project.group' are specified.
Merge remote-tracking branch 'private/issue/368-M1' into 0.6-develop
Fix Gradle plugin for eclipse command.

Unset mandatory properties should be ignored when
configuring projects for Eclipse workspace.
Merge remote-tracking branch 'private/issue/368-M1' into 0.6-develop
Fixed #371.

Resolve variables on Direct I/O output base path.
Merge remote-tracking branch 'private/issue/371' into 0.6-develop
Improves javadoc task (#368).

* Fix default conventions in 'javadoc' task.
* Fix some javadoc options.