add --dl-cache flag for caching downloads

This adds '--dl-cache' flag to the importer.
If provided, that directory for the "workdir" of the source information.
That directory is where all the .dsc and source files are downloaded to.
If not provided, a temporary directory will be used.

Prior to this change, downloads would be made into the provided directory
(--directory).  This represents a change, in 2 ways:
 a.) that now by default the downloaded files will be removed on exit.
 b.) the temporary files are downloaded to TMPDIR
     This could be problematic since the downloaded files can be large.
     If TMPDIR was a tmpfs or just had limited space, then we might
     cause failure where the --directory argument would work.

To account for both of the above, it might be better to use --directory
argument as the default rather than tmpdir.

# Please enter the commit message for your changes. Lines starting
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# Date:      Wed Nov 2 13:32:30 2016 -0400
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# On branch download-cache
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# Changes to be committed:
#	modified:   usd/SourceInformation.py
#	modified:   usd/importer.py
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#	usd/GitRepository.py.orig
#	usd/SourceInformation.py.orig
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