Using SPDX standard names and IDs for licenses

Hi, 
Here are some changes for adhering more closely to the SPDX standard.
What I did :
- Changed the names of the files to "[SPDX ID].txt"
- In the files, changed the value of "title:" to [SPDX name] and the value of "permalink:" to "/licenses/[SPDX ID]/"
- Updated the links to /licenses/GPL-3.0/ in LGPL-3.0.txt "description:" and "how:" sections.
Points to notice : 
- the "no-licence.txt" file was left as is.
- For the GPL family, I didn't take into account the "only" or "or later" variations.
Cheers, 
Camille

Fixes https://github.com/github/choosealicense.com/issues/64. Fixes https://github.com/github/choosealicense.com/issues/43.