Add support for downloading entire databases (premium)

[DannyBen]

Quandl allows downloading the entire database, for premium databases.

This method is now also supported in PHP Quandl, with the getBulk( $database, $path ) method.

Implementation notes:

• Should work properly with either simple download or curl (auto detected behind the scenes, like the regular download methods).
• This method does not use cache handler if provided, there seems to be little reason to do so.
• Tests for the new method are included as optional (skipped by default). They can be turned on by setting an environment variable (see the README in the tests folder for more info).

This PR comes to at least partially address #10