Porting to BlenderBIM

[brunopostle]

The intent is that something like this add-on should be merged into BlenderBIM:

• [x] The code is not structured like a BlenderBIM module, so there is some refactoring needed.

• [x] There are a couple of dodgy bits of code, mainly relating to the ifcgit_repo global.

• [x] GitPython is easy enough to bundle into the BlenderBIM package as it is all python, however it does require the git executable in the PATH, I think initially this should be an additional step to install separately. Or the add-on could switch from GitPython to dulwich which has a python git implementation,

• [ ] ifcmerge is a single file perl script with no library dependencies, so it should just work on Linux and OSX, there is a windows ifcmerge.exe, but wherever is comes from, it also needs to be installed in the PATH. A solution would be an ifcmerge rewrite in python by somebody who understands python regular expressions, so it can be called directly not as an executable.