Incompatible with IntelliJ IDEA 12

* upgraded IntelliJ version to 12.0.2
* upgraded Sonar version to 3.4
* removed obsolete API methods
* made credentials optional in case they are not needed/ set

Issues: https://github.com/gshakhn/sonar-intellij-plugin/issues/7
reformatted code
Does not use proxy settings

* added checkbox to indicate whether proxy settings shall be used

Issues: https://github.com/gshakhn/sonar-intellij-plugin/issues/8
Use IDEA's proxy settings

* extracted "getSonar()" method from all components to SonarUtils
* made all components use the same implementation of getSonar
* refactored getSonar method to check whether to use IDEA's proxy settings
* added checkbox to indicate whether proxy settings shall be used
added ZIP of the plugin which can directly be installed into IntelliJ IDEA
* updated README file
Merge pull request #8 from aheusingfeld/master

Fix for issue 7: Incompatible with IntelliJ IDEA 12