Use ipython configuration system

Use the ipython configuration system.

This allow for the configuration parameter to be introspectable and settable with the %config magic.

e.g:
     In [3]: %config SqlMagic
    SqlMagic options
    --------------
    SqlMagic.autolimit=<Int>
        Current: 0
        Automatically limit the size of the returned result sets
    SqlMagic.style=<Unicode>
        Current: u'DEFAULT'
        Set the table printing style to any of prettytable's defined styles
        (currently DEFAULT, MSWORD_FRIENDLY, PLAIN_COLUMNS, RANDOM)

    In [4]: %config SqlMagic.autolimit = 1000


I had to made some change to the test architecture for this to be cleaner,  it is now more similar to the other extensions. I run the tests with : ipython -c "import nose; nose.run()"  I hope this is compatible with your workflow