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.
Can we get that value lower? The timeout isn't for the whole download and installation time, it's for "progress doesn't move", meaning that if the download progress is stuck for 480s at the same percentage, which shouldn't happen even on really slow network, isn't it? (same for pycharm)