Regarding issue tracker #29442 - Backport cURL support and related updater and installer fixes

This pull request fixes a long standing Joomla! 2.5 issue which prevents any updates (core Joomla! and its extensions) from working on servers where URL fopen() wrappers are disabled. Moreover it fixes a related issue with installing extensions from URL on such servers.

The changes made are:

* Backport of JHttpFactory from Joomla! 3
* Modification of JUpdater and related classes to use JHttpFactory
* Removal of evil eval() code in JUpdater and related classes which caused the updater to not work at all on shared hosts which correctly and justifiably disallow the use of eval() for security reasons
* Modification of JInstallerHelper::downloadPackage to use JHttpFactory

This Pull Request does not have any backwards compatibility issues.