[low prio] Create branches for old versions

[jheeffer]

Old versions of CDIAC--co2 data are available here:

http://cdiac.ornl.gov/ftp/ndp030/CSV-FILES/

Create branches like /release/1751-2008/ for older versions. Current version is in master branch. Be sure to update definitions and version in readme in each branch.

There might be some inconsistencies with definition of total emission in different versions. Watch out.

[jheeffer]

This might conflict with our own versioning which we'll implement. When fixing this issue, make sure there's no conflict.