Unified Reader / Able to pass file paths to the irispy objects

[nabobalis]

There are two seperate readers for IRIS data, the goal would be to have one reader that can figure out what the input file is and return the correct irispy object (similar to how sunpy does it).

The other thing would be to pass in a file path into the irispy objects and have it return instead of asking for a (data,wcs) pair (similar to how sunpy.map does it).

Original issue:
sunpy/sunraster#80

It would be nice to have a generic load function to load any kind of IRIS file into the appropriate objects. E.g.:

    data = irispy.read_iris("some_file.fits")

And the type of data would be either an `IRISSpectrograph`, `IRISSJICube` or other appropriate class.
This could work for any level file (assuming there are loader functions for all) and any instrument, since this information is all in the header file.
This  would be optional and would not return a new class, but just redirect to the appropriate reader.
It could also made future-proof by allowing it to read formats other than FITS if necessary.
It is similar to what the IDL object interface does, and would be natural for people switching from IDL.