Quick and dirty implementation of really executable assembly
Addressing changes suggested by Eugene:

* Changed option name to prependShellScript
* Added new sbt SettingKey called prependScript to allow overridding the default value
* Quoted $0 in default script
* Added try/catch to warn on error running chmod +x
* Use cacheDir to create a temp file rather than File.createTempFile
Forgot to delete tmp file if it exists already
reworked to use AssemblyOption for both enabling and defining the script to prepend
Merge pull request #97 from tlockney/add-real-executable-option

prepend shebang to output jar file