More flexible with shapefile filetypes, in line with the docs

Fixes #35.

The docs for the reader say that no exception is thrown if any of the filetypes are missing until an operation is called that depends on that file, and that the .shx file is optional. This was only the case when giving file-like objects for each filetype, not when specifying the filename. And missing .shx files raised errors. 

This commit allows the same flexibility when specifying filenames, with the added benefit of reading standalone dbf files.

Author: karimbahgat

shapefile.py

@@ -259,23 +259,24 @@ def __init__(self, *args, **kwargs):
     def load(self, shapefile=None):
         """Opens a shapefile from a filename or file-like
         object. Normally this method would be called by the
-        constructor with the file object or file name as an
-        argument."""
+        constructor with the file name as an argument."""
         if shapefile:
             (shapeName, ext) = os.path.splitext(shapefile)
             self.shapeName = shapeName
             try:
                 self.shp = open("%s.shp" % shapeName, "rb")
             except IOError:
-                raise ShapefileException("Unable to open %s.shp" % shapeName)
+                pass
             try:
                 self.shx = open("%s.shx" % shapeName, "rb")
             except IOError:
-                raise ShapefileException("Unable to open %s.shx" % shapeName)
+                pass
             try:
                 self.dbf = open("%s.dbf" % shapeName, "rb")
             except IOError:
-                raise ShapefileException("Unable to open %s.dbf" % shapeName)
+                pass
+            if not (self.shp or self.dbf):
+                raise ShapefileException("Unable to open %s.dbf or %s.shp." % (shapeName, shapeName) )
         if self.shp:
             self.__shpHeader()
         if self.dbf: