Make jar accept alt. main class as arg. Fixes #897 and fixes #899.

By having jar accepting an alternative main class, uberjar can call jar with an
additional parameter and uberjar's alternative main class won't be ignored. This
fixes #897. In addition, as jar is now responsible for adding main to :aot, the
main class will now actually be compiled. This fixes #899.
Don't conj :main during uberjar if :aot is :all.

When :aot is :all, don't conjoin the :main value to :aot, as this will result in
an error.
Merge pull request #902 from hyPiRion/uberjar-compile-after-aot-calculations

Make jar accept alternative main class
Move contributing guidelines into their own file.
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Clarify the exit docstring.