Initial JetBrains support - Idea and PyCharm Community Editions.

Work in progress, do not merge yet.

DownloadCenter grew support for custom headers in DownloadItems.
Support for PyCharm and Idea Community Editions.

Need to add tests and refactor the JetBrains stuff a little so more logic is in the base JetBrains installer - this will allow us to add support for more IDEs easily.
It would be nice to have a small test for that new case. Most of those tests are in tests/small/test_frameworks_loader.py and fake frameworks list in tests/data/

I can give a hand if needed, of course :)