Adds the ability to download DBs via the API.

[typhonius]

No description provided.

[coveralls]

Coverage increased (+0.004%) to 98.896% when pulling 6838ab5 on download-db into 60e6bbc on master.

[coveralls]

Coverage increased (+0.02%) to 98.896% when pulling e4de092 on download-db into b20496d on master.

[typhonius]

A consumer would approach this in the following way:

$backup = new DatabaseBackups($client);
$download = $backup->download('UUID', 'default', '1');
$download->rewind();
$contents = $download->getContents();

$tmpFile = tempnam(sys_get_temp_dir(), 'database') . '.sql.gz';
file_put_contents($tmpFile, $aliases, LOCK_EX);

[weitzman]

I'm seeing a curl error shortly after the the database starts downloading. Its working for you? Is your DB large? Some relevant discussion of this error guzzle/guzzle#1600 and https://stackoverflow.com/questions/1759956/curl-error-18-transfer-closed-with-outstanding-read-data-remaining/1847591

cURL error 18: transfer closed with 1755079179 bytes remaining to read (see http://curl.haxx.se/libcurl/c/libcurl-errors.html)

[typhonius]

With further investigation, the above issue looks like it was limited to a single subscription so this PR should work.

[weitzman]

Yes, PR likely works. My subscription still doesn't work sadly. The engineers are "investigating"

[typhonius]

Merged in 2afdfb3