Config for nightly builds

For enabling nightly builds this branch changes these things:

- the project setup is fixed for being called from Xamarin command line build tools (and being independant of the local shell)
- the Release and the Debug configurations are fixed to represent, what the names indicate
(which configuration will be used for which kind of build is another discussion)
- a bugfix for SparkleShare not to crash when being built without debug sysbols (The NSImage obtained using ImageNamed must not be modified!)

// I configured an automated build on my branch, the result can be downloaded here: 
// https://www2.junidas.de/download/SparkleShare-nightly.tgz
// This download is not yet fully functional, but I am working on it

I can reconfigure this to the master branch of your project, when the fixes are merged. I could upload the artifact wherever you would like to have it.

I will go on adding an identification for the nightly builds in the About panel, so on log files, crash reports and screenshots we will be able to identify the build.

Markus