fetchR v2.0-0

Major changes

• fetchR is now generalised to incorporate SpatialPolyons representing any
  marine location on Earth (#6). As a result, fetchR 2.0 is no longer limited
  to calculating wind fetch within New Zealand coastal areas.

Rewrite the entire algorithm for calculating wind fetch.

• The rgeos::gBuffer function is used to calculate the end points of the fetch
  vectors at their maximum distance.

• The interactions between the fetch vectors and the coastlines, or any of the
  polygons, are calculated with the rgeos::gIntersection function as opposed to
  the slow, iterative algorithm used by the package's predecessors.

• Computation times are dramatically decreased.

Minor changes

Create a vignette for the package; vignette("introduction-to-fetchR").