Does not work with different coordinate systems

[SrNetoChan]

[cbertelli]

As said on issue #5, the user is not supposed to use geographic coordinates, but metres. Only projected coordinates give appropriate results. I tried with both layers in WGS 84 coordinates (very common I think, expecially if you use OSM or GPS data) and I got an unexpected results (really short times, say 0.0000000000023 minutes). Maybe more instructions would be enough to help the user obtaining good results.