Cleaned up and rebased "libdivecomputer-NG" branch

[Dirk Hohndel]

The libdivecomputer and Subsurface changes have now been pushed to master.
I've also updated the documentation to refer to this as the Subsurface-NG branch.

We should now focus on figuring out the problems with the Shearwater downloads.
For some people these fail on rfcomm, for some they fail on BLE but succeed on
rfcomm, and it seems that some can't download from their Shearwater devices at
all...

/D