Make neuron name serivce a singleton

Up to now the neuron name service was instantiated only once and stored in a
global variable. This, however, isn't very robust. This commit makes the neuron
name service a singleton which can be retrieved using

    NeuronNameService.getInstance()

Getting rid of the global variable also allows easier minification of this code,
in case we want to use this.
Remove CSS files for Internet Explorer 5, 6 and 7

Currently we don't support Internet Explorer at all and some features and
libraries we use are not supported in IE 5, 6 or 7. For instance, Array's map,
reduce, filter, etc. functions are introduced in IE 9. Libraries like Cytoscape
and parts of CATMAID use the canvas element. Support for it was introduced in IE
9 as well. The 3D viewer uses WebGL, which was introduced in IE 11. Therefore,
there is certainly no need for CSS files that improve support for IE 5, 6 and 7.