Merge pull request #169 from dmcglinn/master

Add a command line usage section to the  README file
Add a template for downloading a file directly and copying it to the desired directory
Add an engine for only downloading a file

At the moment this is just a dummy engine since it doesn't really
need to do anything.
Allow downloading of files without cleaning line endings

To allow us to use engine.download_file for directly downloading
non text files it is necessary to be able to override its line
ending cleanup.
Have the download only template bypass line ending cleanup
Report if a file is now downloaded because it is in the working directory

This is an edge case for the DownloadOnlyTemplate where the file
is in the current DATA_DIR, but not in the appropriate raw_data
directory.
Add a script to download the mammal super tree
Adding script for the Bioclim 2.5 square km data
Adding 'retriever download' alias to 'retriever install download' for simplicity.
Replace all non-downloading methods in download_only engine with a dummy method.
After running download_only script, copy all files into the home directory. Moving this step from bioclim script to the engine.
Overloading some more methods for download_only engine to correctly handle downloads.
Get rid of insert_data_from_file.
Refactoring try_install_all to take an optional list of engines to test.
This set reset isn't needed and might mess things up. Keep track of all found files until final_cleanup is called.
Removing print statement.
Adding path option to download engine.
Fail bioclim download on engines other than 'download only.'
Merge pull request #1 from bendmorris/download-engine

Download engine modifications
Merge pull request #173 from ethanwhite/download-engine

Add ability to directly download files with no processing