Fix Gem.ruby singleton method on Windows

This issue was identified when trying to install the *murmurhash3* gem, which includes native extensions, on the Windows platform using the jruby-complete JAR file as the executable. This generated an error message as follows:

    C:\temp\JRuby_Test>set GEM_HOME=C:\temp\JRuby_Test\vendor\
    C:\temp\JRuby_Test>set GEM_PATH=C:\temp\JRuby_Test\vendor\
    C:\temp\JRuby_Test>java -jar C:\temp\JRuby_Test\jruby-complete-1.7.10.jar -S gem install murmurhash3
    Building native extensions.  This could take a while...
    ERROR:  Error installing murmurhash3:
            ERROR: Failed to build gem native extension.
    
        java -jar /C:/temp/JRuby_Test/jruby-complete-1.7.10.jar extconf.rb
    Error: Unable to access jarfile /C:/temp/JRuby_Test/jruby-complete-1.7.10.jar
    
    
    Gem files will remain installed in C:/temp/JRuby_Test/vendor/gems/murmurhash3-0.1.4 for inspection.
    Results logged to C:/temp/JRuby_Test/vendor/gems/murmurhash3-0.1.4/ext/murmurhash3/gem_make.out

After digging in a bit, it looked like the problem was that there was an extraneous slash at the beginning of the result of the Gem.ruby singleton method call, causing the command to come out as `/C:/...` instead of just `C:/...`

I figured while I was in there, I'd extract the `Gem.jar_path` method and document what it's doing since it wasn't obvious to me at first glance what the "jarred_path" input string was going to look like.

**Full Disclosure**: I didn't run all the unit tests myself to make sure things don't break because I don't have the whole JRuby development environment set up on my machine. I did, however, manually test the specific behavior related to this change on Windows 7 using both the system-installed jruby.exe and the jruby-complete.jar file, and also on CentOS 6.5.