Allow staged installation + import Rcpp::sourceCpp() + fix error in ConvertHexToBytes

[elipousson]

I'm not sure if I have this all done right but I think this pull request may close out #4 and potentially could help with #6. Here are the key changes:

• Added Rcpp::sourceCpp() to imports (following guidance from Advanced R on using Rcpp in a package)
• Restored default setting for staged installation
• Corrected what appears to be a potential typo in checking for odd-length vectors (swapping | for == in utilities.cpp)

Building the package still returns a variety of warnings about the C++ code. I was able to address a couple obvious issues (removing unused variables this_compound_index and MAXPAGE) but I don't really know enough C++ to fix the other issues.

I also added a NEWS.md file to keep track of changes.

[AllanCameron]

Hi @elipousson. I have fixed the C++ code so compilation should go ahead OK. The checks seem to work out, except I had to reinstate the no-staged compilation as this was causing an error. The ConvertHexToBytes function is correct - I know it doesn't look it, but it uses a bitwise OR to check whether the vector length is odd or even. Using == is wrong here. I think the staged installation problem has to do with the way the pdf paths are stored. I'll have a look at this and try to fix it.

[elipousson]

Glad you had a chance to get back to this, thank you! I'll close this PR since you already incorporated the changes. Looking forward to using the package more in the future.