Add footprint class to normalize footprint representations.

[hardkrash]

Currently footprint names are hard coded in the programs.

The footprints should be converted into a class that is then resolved based upon the tool being output into. Optionally we could also output the source used to make the footprint for the layout tool.

R_0402_info = {
    KICAD: {'footprint': 'Resistors_SMD:R_0805'},
    PCB_GEDA: {'footprint': "0805",
                           'footprint_source': "Insert M4 footprint source code here.",
    },
}

R_0402 = Footprint(R_0402_info)

I seem to recall that there was a standard for industry standard footprints. http://www.ipc.org/ContentPage.aspx?pageid=Land-Pattern-Calculator Not sure of it's use but they claim support for kicad.

[hardkrash]

To support intelligent footprint selection we should add footprint properties like dimensions etc...

This is to enable if footprint_a >= footprint_b: