Merge pull request #10 from aupiff/master

basic latex functionality
creating functions for each class to generate a latex representation of class contents, abstracted call to pdflatex via subprocess, added removal of unwanted .log and .aux files that are generated during pdf-creation
removing stupid calculation stuff in def.tex and removing the file entirely. simplified output file generation, same code for html and latex
fixing module import in test_envois.py, removed bit in readme about using default latex template, starting to write my own
removing currency class, updating latex template to display expenses in longtable format so expense tables may span multiple pages
changing invoice.html to handle recent changes to how currency is handled
improved latex template, using individual members of company.address instead of .__self__() and .latex() functions--I think this is cleaner