Do not call curl-config --static-libs if --libs succeeded.

Fixes #52
Get rid of shell syntax, use subprocess appropriately
Drop optional parens
Replace os.popen with subprocess for curl-config --version
Replace os.popen with subprocess for curl-config --cflags
Use descriptive variable names
Use subprocess for curl-config --features call
Explain curl-config options in readme
Add a changelog entry
Merge pull request #54 from p-push/setup-libs

Use --libs only in setup.py if it works, refactor, add documentation