Fix installation failure on Windows

Make setup.py explicitly specify encoding of files it opens.  Without
that, installation fails on Windows with the following error message:

	Collecting pygtrie
	  Using cached pygtrie-2.1.tar.gz
	    Complete output from command python setup.py egg_info:
	    Traceback (most recent call last):
	      File "<string>", line 1, in <module>
	      File "C:\Users\WANNAP~1\AppData\Local\Temp\pip-build-553d519x\pygtrie\setup.py", line 17, in <module>
	        readme = f.read()
	      File "C:\Users\wannaphong\Anaconda3\lib\encodings\cp874.py", line 23, in decode
	        return codecs.charmap_decode(input,self.errors,decoding_table)[0]
	    UnicodeDecodeError: 'charmap' codec can't decode byte 0x99 in position 933: character maps to <undefined>

	    ----------------------------------------
	Command "python setup.py egg_info" failed with error code 1 in C:\Users\WANNAP~1\AppData\Local\Temp\pip-build-553d519x\pygtrie\

This fixes <#16>.

Author: wannaphong

setup.py

@@ -2,6 +2,7 @@
 import os
 import re
 import sys
+import codecs
 
 import version
 
@@ -13,9 +14,9 @@
               '-Dversion=' + '.'.join(release.split('.', 2)[0:2]),
               '.', 'html')
 
-with open('README.rst') as f:
+with codecs.open('README.rst', 'r', 'utf-8') as f:
     readme = f.read()
-with open('version-history.rst') as f:
+with codecs.open('version-history.rst', 'r', 'utf-8') as f:
     readme += '\n' + f.read()
 
 kwargs = {