rebased commits:
- Added test case for bi-directional OneToOne in YamlExporter
- Only inspect joinColumns for owning side in bi-directional OneToOne in YamlExporter
- Adding bi-directional test case without joinColumn to XmlExporter test
- Same testcase also applied to PhpExporter
- Fixing bi-directional issue in PhpExporter when inspecting joinColumns index
- Implemented @Ocramius suggestions
ea788fb734