Added the ZF library install feature

Added the possibility to install the ZF library in a specified path.
The command syntax is as follow:
zf.php install zf <path> [<version>]
The <version> can be one of the tags (only the numbers) in https://github.com/zendframework/zf2.
For instance for the tag 'release-2.0.6' the version number is '2.0.6'.
More information in the README.md file.