Automatically load stdlib extensions with approprieate JDK

[altavir]

Currently, only baseline stdlib is loaded automatically when notebook starts. Since kernel knows which JDK is used, it would make sense to load JDK7/JDK8/etc extensions automatically.

In particular, If foun out that I do not have access to Path extensions introduced in stdlib-jdk7 in basic notebook. It is doubly surprising as gradle setup does that automatically nowadays.

[ileasile]

I don't like the idea of adding automatically something you maybe will not ever use... But I'll add a method that will allow to add these extensions in one line, it will work like this.

[altavir]

The thing is that Gradle already does that. So it is about consistency.