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.
We avoid duplicates urls thanks to that logic. There is even a test for it:
"we only do one download when the same file is requested more than once in the same request" in test/small/test_download_center.py (line 267, in test_download_same_file_multiple_times) which is now failing.

However, I agree that you can come with a better syntax to avoid the duplicates :)