Update setup.py
Fixed typo
Inform user that 'developer' command was used as default
Using 'build' instead of 'develop' ('develop' doesn't build extensions)
Using 'develop' option again but turning ON the extensions build flag
Using double quotes for natural language messages
Merge pull request #1123 from DavidRagazzi/patch-3

Uses 'develop' option if no option is passed to 'setup.py'