Add a settingKey to  build separate project of the src/main and src/test

Add a testProjectDirectory settingKey which can build separate project of the src/main and src/test. The purpose to add this key is that, if we has a macros project, the official sbteclipse put the test code and the main code in the same project build, which is a wrong behavior.
The description for a key should be a noun.
e.g. `"The directory where .classpath and .project files for the test project will create"`