Improved document generation in songbook.py.
Clean up some code to be more pythonic.
New format for template parameters.
Default template parameter type is now string.
This release now uses JSON coded data to represent a songbook. Songbook are stored in .sb files. The generation of the corresponding .tex file is done by the songbook.py program and uses templates stored in templates/. The default template is songbook.tmpl. It produces songbooks similar to the previous version of the songbook software suite.