Join user and package area files

[djhoese]

Currently the parse_area_file function only allows one single file. This causes limitations in software like satpy where we may want to have a "user" area file and a "package" area file. The easiest/cleanest way to handle this is to make a pyresample allow more than one file. This is even more useful for the new yaml files which could allow users area files to "update" existing information in the "package" area file.

I propose that there be a new function (for backwards compatibility) that accepts one of the following input formats for the areas files:

1. single area file name
2. single file-like object
3. single file content string
4. iterable of area file name or file-like object

[djhoese]

Fixed by #63