Add support for detecting cray-xe and cray-xc platform.

Look for CLEinfo config file, read it, and inspect the NETWORK value. If it is
"gem", set platform to "cray-xe". If it is "ari", set platform to "cray-xc".

If the CLEinfo file does not exist, or if it does not contain a known NETWORK
value, default to the old platform logic.

### TODO:
* [x] test on XC
* [x] test on XE
* [x] test on eslogin node
* [x] test on CCS
* ~~[ ] if possible, test on XT (will have unknown NETWORK value)~~
* [x] test on regular platforms to make sure old defaults are ok (verified on darwin, linux64)
* [x] check README.cray for doc updates