the large reorganisation

now the directory structure is nice, but nothing is functional
rename bolt to bolttools

this avoids import clashs
started to overhaul the blt spec

and incorporate lessons learned. The new spec will hopefully allow for
a clearer library structure and nicer code.
rewrote blt parser

following blt 0.2 spec. With much better error checking and a test
suite. Decided to not pay attention to compatibility, gets a mess
otherwise.
changed classes into associative array

the ids are needed to reference classes from the outside in a stable
manner. Also some more explanations and minor changes.
fixed some bugs and adapted parser to changed spec
more refinements to the specs and OpenSCAD backend
added forgotten images for spec
changed classes storage back to lists

this is more convenient for the changed spec
updated tests
first commit of openscad backend
removed unused argument of write_output
added check for forbidden collection ids
split classes with many standards during parsing

this makes things easier in the backend code and allows for clean
treatment of standard collections.
fixed standards in parser and added them to openscad backend

fixes #21
big reorganisation and top level tool

be consistent in that the Repo is the main data structure, that holds
the backend data. Split Backend into Exporter and Data. Many other
changes and fixes. OpenSCAD backend should work, FreeCAD not.
improved robustness of bolts main commandline tool
more and better checks for blt files
suppress baseless classes in OpenSCAD output
commandline tool complains about wrong incantation