User-Agent header improvements

1. Currently the default User-Agent header is of the form `createsend-python-{version}`. Modify this to include Python version and platform information: `createsend-python-{version}-{sys.version_info[0]}.{sys.version_info[1]}.{sys.version_info[2]}-{platform.platform()}`.
    So the default user agent would look more like:
    ```python
    "createsend-python-3.1.0-2.7.3-Darwin-12.3.0-x86_64-i386-64bit"
    ```

2. Allow the default User-Agent header to be overridden by doing this:

    ```python
    CreateSend.user_agent = "my custom user agent"
    ```