Project renaming

[stariy95]

Name of choice for this project is JJava, need to review and change it where needed.
Also we need to keep in mind compatibility at the Jupyter level, so it won't require to change existing code.

[andrus]

Hey guys, @stariy95 @m-dzianishchyts , we just renamed all the env vars from IJAVA_ to JJAVA_, which is fine, but I suggest that we support the old vars as well for some time as deprecated. I would like JJava to stay compatible with iJava for some time.