Add the new Retriever download only functionality

The Retriever now supports the direct download of raw data files
without any manipulation. This can be useful for full
reproducibility and provenance tracking and allows non-tabular
data to be downloaded using the retriever (e.g., geospatial data).
Merge pull request #8 from ethanwhite/add-download

Add the new Retriever download only functionality
Change required R version from 3.0.2 to 3.0.0
Add system requirements argument to the description file that lists the dependency on the EcoData Retriever v 1.6 or great being installed.
Fix typo in README.md
Improve the documentation of the package that is pulled up by ?ecoretriever using the text found in README.md
Add instructions for how to add Retriever to the path to README.md
Minor correction to the help file for the function install_data()
Fix typo in ecoretriever help file.
Minor changes to the help file for the download_data() function.
Add a fetch() function as requested in #9
Add the fetch() function to the NAMESPACE file.
Add a help file for the fetch() function
Add a quiet argument to the fetch() function to suppress retriever output messages.
Add an example of the function fetch() to the README.md file.
Implement a more secure method of password transfer via a conn file in response to #7.
Add .gitignore file so that database connection files .conn files are not accidently commited.
Change the order of the conn_file and log_dir arguments that can be supplied to the install_data() function
Update help file for install_data() due to the usage of the conn_file argument.
Improve how the conn_file argument is handled and error checked by the function install_data() per comment by @ethanwhite on #12