Improve Cygwin package

Things I’ve done in this PR:

* **Revert package ID back to original** (to prevent issues with case sensitivity). I highly recommend to not change the package ID of NuGet/Chocolatey packages once they are released, not even changing uppercase to lowercase. NuGet’s package IDs **are** case sensitive. I already made bad experiences with this. Once I changed the package ID of a package to lowercase (can’t remember the name), but then this happened: I installed another package which had the package with the lowercased ID as dependency. But instead of installing the latest version of the dependency package, it installed the previous version where the package was still uppercase (= original). So it seems that changing package IDs to uppercase/lowercase partially breaks dependencies.
* **Convert to automatic package**
* **Change installation folder to lowercase “cygwin”.** Cygwin’s default installation folder is also lowercase. In addition it is uncommon in UNIX-like environments to have uppercase program folders. I wouldn’t break with this de-facto standard. Folks who already have “Cygwin” as installation folder are OK anyways (due to Windows’ path case insensitiveness).
* **Remove "-N" argument.** Why not creating a start menu entry? It does not hurt anybody.
* **Make use of "Get-BinRoot"**

After you merge this, don’t forget to do the following steps:
* Make the “chocolatey” user a co-maintainer of this package.
* Update repo on the server which handles these automatic packages.
* Import the job the Cygwin package into Ketarin. I prepared it for you: https://gist.github.com/TomOne/d25690dc6e5793e029dc