Many improvements

[r-barnes]

I've been working with the code a bit and introduced some improvments:

• Modifications to suppress compilation warnings due to signedness.
• Cleaning of README.md file
• Modifications so code can be compiled with the C++11 standard rather than the C++1z standard, allowing for usage in a broader range of development environments.
• Introduction of link-time optimization, which may lead to improved performance.
• Inclusion of the -g flag in compilation, since this doesn't affect speed, but does improve debugging capabilities.
• Introduction of a single script RUN.sh which performs all the operations described in the README.md file.
• Movement of licensing info to its own file.
• Addition of a script extract_district_boundaries.py which exports districts to a form compatible with standard GIS software.
• Increasing the number of iterations before the program raises a "failure to converge" message.
• Use of the const modifier at various points in the code.
• Command-line argument checking in do_redistrict.cpp.