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.
fix typos in chpl_platform
Update README.cray to reflect platform detection on XC and XE.
Merge branch 'master' of github.com:chapel-lang/chapel into better-detect-cray-platform
Fix range
Merge pull request #630 from thomasvandoren/better-detect-cray-platform

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.