(fixes #47) Read message to send from file

This allows sending Unicode messages from files, avoiding encoding problems with Python 2.x (on Windows it does not know how to process cp65001, the encoding Windows uses to process Unicode strings)
Test by saving the following string as msg.txt in UTF-8 without BOM format, then run yowsup-cli -c YOUR_CONFIG_FILE -s YOUR_PHONE_NUMBER msg.txt -f

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