Timing benchmarks

Timing benchmarks are now in `./tests/timing-benchmark.R, running which produces this output:

SF: Median times (in ms) for ( points lines multilinestrings multipolygons other_relations ):
        ( 157.9642 161.3807 152.0374 153.3336 159.6488 ); total =  784.3647 
osmdata: Median time =  361.1614  ms

osmdata took  0.4604508  times longer to extract  42.18363  times as much data

And note that it really does extract more useful data, as detailed in this wiki page.