Reorganize folder structure to make it more usable

[pavanmr94]

Hi,

Thanks for publishing pkcs11 in a git repo.
We would like to consume pkcs11 directly from this repo in our project, and during initial investigation, we found a few less than ideal things in the structure of the repo:

• You have both the stable version and the working set in the same branch. It would be a lot cleaner if the working set was in a different branch, which would leave the master branch clean.

• There are the version names in the folders. It would be really helpful if you could move it to some structure agnostic of version number, such as:
  include/
  |-- pkcs11.h
  |-- pkcs11t.h
  |-- pkcs11f.h
  identifier_db/
  |-- xyz..

  This would greatly help when upgrading versions as we can just update the submodule and don't have to change all our build scripts to point to a new location.

• If you could have a release tag on a published version, it would help us to base our projects on a known stable point in time, rather than just pointing to master where the folder structure could change anytime.

These changes would also help others who want to submodule this directly into their projects.
Feel free to ask for clarification if anything is not clear.

Regards,
Pavan