GOES lightcurve now knows which satellite to download from and the data type

Here's an attempt to address #609. I added a function that contains a dictionary of the GOES satellite operational dates and available data types (this information comes from the GOES API http://www.ngdc.noaa.gov/goes/sem/getData). 

The function checks against the requested time for retrieving GOES data and finds out a) the newest available GOES satellite and b) the data class. The API query is then carried out using these.

Not sure if having a dictionary like this within SunPy itself is the best solution (maybe it should grab this information from the GOES site), but I figured the operational dates for GOES wouldn't change much!