created SPARQL provider

still very much incomplete
refactored presenter

now only distinguishes two node types (entity vs. collection) and two
edge types (directed vs.  undirected), though both remain extensible
improved store API, returning newly added nodes

seems sensible that way, though the enter thing needs more documentation
clarified translation processing
added labels for neighboring nodes

this is pretty awkward and hacky for the moment (cf. inline comments) -
hopefully it will be cleaned up eventually
made entry point configurable via frag ID

for development & demonstration purposes
turned undirected edges into dashed lines
refactored node color selection for consistency
increased max. zoom distance, added cautionary comments
tweaked link distance and strength, distinguishing directed & undirected
added preset entry points for demo purposes
removed obsolete node type definition

"entity" is the default
restructured internal cache handling

in preparation of edge caching

plus moved validation to lower-level function
ensured duplicate edges are discarded

the current definition of edge identity (sourceID + targetID) might be
too strict (cf. inline comments) - to be determined...
increased link distance based on node weight/degree
ensured page is updated if frag ID is changed

crude, but effective
added loading indicator

very much preliminary, styling-wise

cf. http://bl.ocks.org/1667139
removed obsolete CSS dependency
added zoom effect for node under cursor

enlarged nodes allow more comfortable manipulation
removed zoom effect for hover-selected nodes

unfortunately, this led to weird effects while dragging nodes

this partially reverts the previous commit
(e318aa322e660d6c7ab2051a78b5515ea64be08a)