HttpClient is embedded in the version 0.3 of the jenkins plugin and works fine for IDEA 8.x. Unfortunately, the version 9 10 (and more) of IDEA causes critical bug during the plugin startup because this HttpClient is now packed within the IDEA. So Java does not understand why 2 same libs are in the classpath and causes an InitializationError.
I will create a specific Intellij 9- branch for supporting old versions. This way will also allow me using Intellij SDK 10+ and JDK6.