Make intellij plugin aware of proto source dirs

[zpencer]

Verified that with this change, the intellij proto plugin no longer complains about import targets being not found. Jump to definition works as expected in .proto files.

Fixes #156

[zpencer]

Need to verify that protobuf plugin failing will not impede intellij from opening the project. I need the idea task to run after the protobuf plugin, but it doesn't actually need the plugin to succeed.

[zpencer]

@zhangkun83 PTAL

This PR modifies the configuration step of the GenerateProtoTask so that all known proto directories are added to intellij's sourceDir/testSourceDir sets. Because this is the configuration step, we know the proto dirs are registered before the idea task is run, and that the actual success of GenerateProtoTask is irrelevant.

There is a slight gotcha that the idea plugin will silently ignore directories that do not exist. The workaround is to create the directories right before the GenerateIdeaModule task runs. We do not want to create the directory at the configuration phase because it will always run and create the directories, even if we are just trying to do a ./gradlew clean.

[zhangkun83]

LGTM except for a minor comment.

[zhangkun83]

Is this thrown from getByType()? Since this method is called without checking the presence of the IDEA plugin, this is considered a normal control flow, and exceptions should not be used for that.

[zpencer]

Looks like findByType the non throwing version that I didn't know existed. Updating PR.