lazy loaded clib

[PeterPetrik]

• introduced pygeodiff.shutdown() to correctly unload on windows
• pulled out context from geodifflib to geodiff
• geodifflib is now shared between all instances of pygeodiff.GeoDiff()
• bumped ver

latest python had some problems with comments with \ so I replaced it with :

fix #205

[dvdkon]

I rebased this PR on current master.

[wonder-sk]

actually...

• is it correct that GeoDiff class calls shutdown() upon its destruction? I think that may cause a series of loads and unloads of the geodiff C library when GeoDiff is created and deleted in sequence - or not?
• and do I understand it correctly that each GeoDiff has its own GeoDiffLib instance? (the PR description says the geodifflib should be shared now?)

[dvdkon]

• is it correct that GeoDiff class calls shutdown() upon its destruction? I think that may cause a series of loads and unloads of the geodiff C library when GeoDiff is created and deleted in sequence - or not?

Hm, I missed that. I think it doesn't make sense, since GeoDiffLib.shutdown() is also called by GeoDiffLib.__del__. But loading/unloading the library when multiple GeoDiff objects are created with distinct lifetimes is probably unavoidable, if the goal is to unload the library when it's no longer needed.

• and do I understand it correctly that each GeoDiff has its own GeoDiffLib instance? (the PR description says the geodifflib should be shared now?)

Yes, this way we'll end up with multiple GeoDiffLib instances (that will clean up after themselves on Windows, but still). I'll commit a fix.