Handle all JavaCompile tasks

[ypoluektovich]

This patch adds more sophisticated handling for Java projects, enabling one to use APT not only in 'compileJava' tasks, but all tasks of type 'JavaCompile'.

Important notes:

• The default generated source directory name has been changed, as has the mechanism of obtaining said default. The new version enables the project build script to read the default, not only override it.
• The default directory change affects both Java and Android projects. All the other changes are Java-exclusive, because I have absolutely no experience with Android. Hopefully, if similar functionality is needed, you (or another contributor) can add it easily based on my work.
• The plugin used to be applied fully inside a project.afterEvaluate invocation. From now on, this is not the case. The reason is that, if the plugin only works after the project is evaluated, the user cannot access configurations and extensions generated by this plugin in the project's build script. To fix that, I moved the creation of configurations and extensions, which are now done at the time of plugin application.
• This pull-request breaks the tests. I can't fix them, because I can't even run them, because I have groovy 2.2.3 and spock wants 1.8 (or 2.0.5 with the latest release of spock, but it's still too low), and I can't make them become friends :(

[ypoluektovich]

In fact, I thought about it some more, and made the second commit which gets rid of project.afterEvaluate completely for Java projects. Now the configuration application task is available to the user in the project build script too.

My own tests (on my project) show that it works fine.

[futtetennista]

Thanks very much for the PR! I'm a little busy atm, but I'll make sure I'll have a proper look at this asap.

[confile]

@ypoluektovich Could you please provide an example how I can use APT on tasks of type JavaCompile?

[confile]

@ypoluektovich I get the following error when I try to use your modification:

* What went wrong:
A problem occurred evaluating root project 'apt'.
> org/apache/commons/lang/StringUtils

[ypoluektovich]

Example 1: https://github.com/ypoluektovich/msyu-logging/blob/master/example/build.gradle
Line 3 is for where I keep my build of this PR, line 4 is to make commons-lang available on APT classpath (you can also use mavenCentral() or your corporate proxying maven repo), line 7 adds the plugin itself, line 11 applies, line 18 adds an annotation processor.

Example 2: https://github.com/ypoluektovich/msyu-logging/blob/master/runtime/build.gradle
Same as above, only this time we need the annotation processor for tests, not for production files. See line 26.