allow use of BOLTS as a python module without FreeCAD
support for common parameters

partly fulfilling #62
added some common parameter specs

To DIN933 for testing, to the bearings because their models are
unbearably slow.
Merge branch 'master' into step
Merge branch 'master' into step
new development snapshot
added pitch data to DIN933 and common params

chamfered washer export is suppressed because of #65, M25 not available
in class
Merge branch 'master' into step
merge branch step of bolttools
moved backend data to backends folder
FreeCAD backend works again
openscad backend works again
moved downloads to fit to new layout
Checker backend

will not be too useful on its own, but will be used by other backends
removed drawings from removed furniture collection
no output and adapted image checks
statistics backend
html backend

strongly rewritten compared to before
adapted blt and drawings data to new blt changes
unified html and downloads backend