Fixed dataset id and download options

[bdnorman]

From solution here #46

[neteler]

Hi @abhilashsinghimd and @MartinPontius, would you be willing to test this change?

[MartinPontius]

@neteler The commit in this PR is basically the same as this commit which was already merged in PR #97. So I think this PR can simply be closed without merge.

As mentioned in multiple issues, the principal problem here is that those hard-coded ids have to be updated from time to time manually. A cleaner and more elegant solution was described here. In short: the USGS/EROS Machine-to-Machine API should be used to obtain the correct download urls.
However, as mentioned in PR #97 some (in this context important) API endpoints are restricted if users haven't specifically requested access to the M2M API. I haven't done so myself, so I can't use these endpoints. It might be possible to get the download urls with the download-retrieve endpoint alone which can be used without special access. At least I can see some datasets with download url in the response. Unfortunately, I don't have time at the moment to investigate this further.