This adds an option to show the result of a calculation in hexadecimal format. But while I was looking into it, I also added some more improvements.
I split them into 6 commits, but if it is easier, I can also squash them into a single commit. I will attach the diffs to this ticket, but you can also add https://github.com/wimh/xfce4-calculator-plugin.git as remote repo (branch dev).

I will be happy to answer any questions. But I will add some comments about the changes anyway;

https://github.com/wimh/xfce4-calculator-plugin/commit/7c9838623dde11068fe3d08750613371596e1a15 autogen.sh contained code to generate the revision from svn. As git is used now, this can be removed.

https://github.com/wimh/xfce4-calculator-plugin/commit/53c7dbf6ba0682bb97b311ea3314c52b8130c42c generate changelog from git log. A changelog does not seem useful to me if it is not updated. If this can be done automatically, I think everybody is happy...

https://github.com/wimh/xfce4-calculator-plugin/commit/c0263d4d190bdc2df491abb40a8b84cee7e8dd62 move test scripts (make check) to separate directory

https://github.com/wimh/xfce4-calculator-plugin/commit/824c13856ce358216e94d7f4c0cc19d4c09134bc adds tests for existing operations

https://github.com/wimh/xfce4-calculator-plugin/commit/e2d629f841375f73c58fcc2fd506bcdb8740ac0e add modulo operator. Note this will make it more difficult if a percent operator (33+10%) will be added in the future.

https://github.com/wimh/xfce4-calculator-plugin/commit/bb25e1581ce9f230eb5a82ce1e84e36cbc72a632 This adds an option to display hexadecimal output. I am not sure if the #ifdef conflicts with any coding conventions. 

Note I did not update the version number (yet).