Added a method to djblet's extension manager that installs an extension, given a remote URL with the extension egg and package name.

This modification is required as a pre-requisite, for the upcoming extension browser module to ReviewBoard.

There could be possible errors such as (but not limited to): The URL being a 404, incorrect package_name, incompatible egg or the package being already present. These would be thrown as exceptions and have to be handled by the caller appropriately.

The changes have been tested with the different variations of extension browser module so far.

Presently, with the module being absent it can tested manually by invoking the install_extension() method on the extension manager with the required parameters. If the extension isn't already installed, the installation would go through.