Add demo with multinomial logistic regression

Squashed commit of the following:

commit 09db4ea25db41e89dcd414f2c823df480629e2a1
Author: Alexander Fabisch <Alexander.Fabisch@dfki.de>
Date:   Fri Aug 23 14:28:10 2013 +0200

    Rename demos to demo

commit 9661951f5b6ad97b0c712ad5b7668161db2a774f
Author: Alexander Fabisch <Alexander.Fabisch@dfki.de>
Date:   Fri Aug 23 10:29:00 2013 +0200

    Optimize and simplify gradient

commit eba5506b3d4f45a99bb2d3cb2ac032190310e993
Author: Alexander Fabisch <afabisch@googlemail.com>
Date:   Fri Aug 23 09:07:25 2013 +0200

    Remove another unused variable

commit f8b4eb0e6b0aa2357ff360ca96190dbaea3f97ae
Author: Alexander Fabisch <afabisch@googlemail.com>
Date:   Fri Aug 23 09:05:52 2013 +0200

    Remove unused variable

commit 9bddcb73851871b270b7d762ecc88e805da4fb00
Author: Alexander Fabisch <afabisch@googlemail.com>
Date:   Fri Aug 23 09:00:46 2013 +0200

    Use softmax from NumericExtensions

    ... and get rid of returns

commit 957615e5b504105312a2ffcab21b648cd1eb6ebc
Author: Alexander Fabisch <afabisch@googlemail.com>
Date:   Fri Aug 23 08:35:53 2013 +0200

    Use indmax and initialize parameters with 0

commit 95f39f7966bb9081d532d365faae3c1c96ba487e
Author: Alexander Fabisch <afabisch@googlemail.com>
Date:   Thu Aug 22 22:03:46 2013 +0200

    Declare types of arguments

commit 910bfdfcb7f298d192cd80d41ad2145ae3111468
Author: Alexander Fabisch <afabisch@googlemail.com>
Date:   Thu Aug 22 21:53:37 2013 +0200

    Use mini-batch SGD to train linear classifier

commit 6ae31dc23b296cbf18aa731cff3721c327c6e27f
Author: Alexander Fabisch <afabisch@googlemail.com>
Date:   Thu Aug 22 21:11:15 2013 +0200

    Clean up linear classifier

commit 634e197b0db2f1761258526abf77de81d8400095
Author: Alexander Fabisch <Alexander.Fabisch@dfki.de>
Date:   Thu Aug 22 16:56:17 2013 +0200

    Add linear classifier with batch gradient descent
Merge pull request #3 from AlexanderFabisch/multinomial_log_reg

Add demo with multinomial logistic regression