Unified browser locator

```
/Library/Java/JavaVirtualMachines/jdk1.7.0_51.jdk/Contents/Home/bin/java -ea -Didea.launcher.port=7532 "-Didea.launcher.bin.path=... org.openqa.selenium.browserlaunchers.locators.BrowserInstallationTest,testBrowserInstallations
/Applications/Firefox.app/Contents/MacOS/firefox-bin
/Applications/Google Chrome.app/Contents/MacOS/Google Chrome
/Applications/Safari.app/Contents/MacOS/Safari

Process finished with exit code 0
```
Test passes with all three browsers located, and switching `assertTrue` to `assertFalse` fails.