Fix clean command: by passing $build object.
imporve version suggestions
Add openssl to default variant
update phar file
Document intl variant
Throw exception if openssl extension is not found.
update release json
Extension installer should output the configure message.

This improves the installation debugging, since the installer isn't
mature enough.
Fix debug option for extension installer.

related issue: #350, #360
Checking in changes prior to tagging of version 1.15.3.
Add more detailed option description.
Rename --alias option to --name option.

An alias should be something link to a real build instead of being the
build itself.
Add --no-compress option to the compile script
lock stability to stable
Extension class refactoring

- Extract logger object dependency from Extension class.
- Add ExtensionManager to run install/enable/disable tasks
ExtensionFactory class to create extension object with its own metadata
Move conflicts info to ExtensionManager class
Fix hasConflicts call
Create a generic Extension class interface
Simplify the extension interface