PyShp 2.1.3

Bug fixes:

• Fix recent bug in geojson hole-in-polygon checking (see #205)
• Misc fixes to allow geo interface dump to json (eg dates as strings)
• Handle additional dbf date null values, and return faulty dates as unicode (see #187)
• Add writer target typecheck
• Fix bugs to allow reading shp/shx/dbf separately
• Allow delayed shapefile loading by passing no args
• Fix error with writing empty z/m shapefile (@mcuprjak)
• Fix signed_area() so ignores z/m coords
• Enforce writing the 11th field name character as null-terminator (only first 10 are used)
• Minor README fixes
• Added more tests