fixed bug 0010694: graph by category broken

• select bug category by id and use bug history referencing
  category field in bug_graph_bycategory.
• fix mapping of category_id in category summary graph

fixed presentation of dates in graph data tables

• use config variable "date_short_format"

fixed setting of alternate font paths in jpgraph

• jpgraph will set a constant on first incluse of the class. After
  this, the path can't be changed. Moved this setting to the includes.