Merge pull request #29 from jan-cerny/master

Change parameter of oscap_generator_set functions on const char *.
Include README.md in tarball
Implementation may refer to the specification

I was tempted to rewrite this code to be independent of xccdf:Benchmark.
However, I have found that the code must depend on xccdf:Benchamark as
Group's score needs to be considered during the calculation.

The comment shall help future generations to determine this quicker.
trac#421: warning: initialization discards 'const' qualifier from
pointer target type