Test astropy.wcs C API

By building a simple C extension and making sure we can import and run
it.
Merge pull request #2464 from mdboom/wcs/add-api-extension-test

Test astropy.wcs C API
Merge pull request #2467 from bsipocz/fix_various_docs_links

Fix various docs links
Minor typo fix in 0.4 what's new

[ci skip]
Fixing documentation links, warnings in wcs
Fixing documentation links, warnings, PEP8 in stat, config, constants