Improved output.

Made the following fields dynamic:
* Invoice number
* Charge type
* Job number
Read company info from book

The gnucash-python bindings seem to lack the necessary functionality to
retrieve company data saved in the book. In the past, I had simply hardcoded
this information in the script. This is dumb, of course, and then my customer
asked me to include my EIN on my invoices, and I did not want to commit that to
github.

The solution was to use ctypes to call the necessary gnucash C API functions.
Make table border slightly less ugly
More graceful err handling

The following error conditions will be handled more gracefully:
1. Invoice not found
2. Invoice without job
3. Book without business data
Improved logging
Updated for GnuCash 2.6.x

Note this commit breaks compatiblity with GnuCash 2.4.x. I have created a new
gnucash2.4 branch from the last version that worked with GnuCash 2.4.x. I do
not plan to continue development on it but I will consider pull requests.
Updated readme for 2.6