Add WKT parser.

Add a parser (read/write) for OGC Well-Known-Text (WKT). It deals with
geometries only, not with features, mostly because WKT can only contain a
single geometry (or geometry collection). This can later on be used to
serialize and deserialize geometries in ol3.
add exports for WKT
address review comments by @twpayne
move last RegExp into constant
Merge pull request #712 from bartvde/wkt

Add WKT parser (r=@ahocevar,@twpayne,@tschaub)