FastDTW seems to be working; not sure how to design tests....

There are some plots that show it more or less makes sense, but I'm not sure what kind of tests I can write to be sure that `DynamicTimeWarp.fastdtw(...)` works correctly. Its components seem well-tested, at least.

Check out the speed results. They show that a 2500x2500 DTW takes 4.5 seconds, while FastDTW with a small radius goes 8x to 100x faster (radius=2 is even faster, but its results are a little suspect).

```
julia> reload("DynamicTimeWarp"); DynamicTimeWarp.dtwspeedtest(2500,1)
Warning: replacing module DynamicTimeWarp
Testing size 2500 radius 40: elapsed time: 0.518089036 seconds (156403384 bytes allocated, 38.30% gc time)
Testing size 2500 radius 20: elapsed time: 0.261834737 seconds (81671320 bytes allocated, 49.82% gc time)
Testing size 2500 radius 10: elapsed time: 0.134307422 seconds (43385704 bytes allocated, 47.94% gc time)
Testing size 2500 radius 5: elapsed time: 0.037186961 seconds (23743664 bytes allocated)
Testing size 2500 radius 2: elapsed time: 0.020199309 seconds (11788280 bytes allocated)
elapsed time: 4.458869655 seconds (1586821752 bytes allocated, 47.75% gc time)

julia> reload("DynamicTimeWarp"); DynamicTimeWarp.dtwspeedtest(2500,2)
Warning: replacing module DynamicTimeWarp
Testing size 2500 radius 40: elapsed time: 0.527473658 seconds (167009480 bytes allocated, 36.64% gc time)
Testing size 2500 radius 20: elapsed time: 0.271713572 seconds (86796408 bytes allocated, 48.04% gc time)
Testing size 2500 radius 10: elapsed time: 0.139185146 seconds (45790120 bytes allocated, 48.59% gc time)
Testing size 2500 radius 5: elapsed time: 0.102216916 seconds (24844432 bytes allocated, 63.74% gc time)
Testing size 2500 radius 2: elapsed time: 0.018097183 seconds (12178104 bytes allocated)
elapsed time: 4.462376547 seconds (1586879480 bytes allocated, 47.69% gc time)
```