Added German (DE) keymap

[mstarostik]

This adds a German keyboard layout and additionally adds a URL parameter layout=xx to specify the keyboard layout to use.

The README.md change already includes the change from #12

[MarcusWichelmann]

Very thanks for writing this keymap, it was quiet a big help for our project! :-)

Here are (the only?) four keys, that seem to be messing in this keymap, for the case that somebody else needs them, too:

    charmapDE['}']   = [[0xE0, 0x38, 0, 0], [0x0B, 0, 0, 0], [0x8B, 0, 0, 0], [0xE0, 0xB8, 0, 0]];
    charmapDE['{']   = [[0xE0, 0x38, 0, 0], [0x08, 0, 0, 0], [0x88, 0, 0, 0], [0xE0, 0xB8, 0, 0]];
    charmapDE['[']   = [[0xE0, 0x38, 0, 0], [0x09, 0, 0, 0], [0x89, 0, 0, 0], [0xE0, 0xB8, 0, 0]];
    charmapDE[']']   = [[0xE0, 0x38, 0, 0], [0x0A, 0, 0, 0], [0x8A, 0, 0, 0], [0xE0, 0xB8, 0, 0]];

I don't have write permissions to this branch, so feel free to add them.
I'm not really a fan of a pull-request for a pull-request... :D