Support subprojects in "infoplist-modify" task

When the project using the xcode plugin is part of a multi-project Gradle build,
the infoplist-modify task fails with file not found error from 'plutil'.
The cause is that the generated path to the .pbxproj file is missing the subproject's directory.
For example, the path /home/username/project/foo.xcodeproj/project.pbxproj was generated
instead of the correct path /home/username/project/subproject/foo.xcodeproj/project.pbxproj

This change adds the subdirectory to the path.
Also tested successfully with a single-project build (no change in path there).
Support subprojects in "test" task

When the project using the xcode plugin is part of a multi-project Gradle build,
the test task fails with file not found error from 'xcodebuild'.
The cause is that the 'xcodebuild' command for tests is run in the current directory.
In a multi-project build the current directory will be the parent directory of the project
when executing a build from the parent project, and not the subproject's directory.

This change sets the current working directory for the test task's 'xcodebuild' command to be the current project's directory.
Also tested successfully with a single-project build (no change in path there).
Merge pull request #60 from perchrh/master

Support for multi-project builds