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.
No, it's just that I thought at the time that would avoid frameworks errors (but we don't have it for now) as the result was a dict. Agree that we can remove this restriction and built a list of DownloadResults respecting request orders. I think a bug logging that would be good.

Meanwhile, maybe just remove the failing test and/or ensure we raise a warning until we get there.