UMN merge

Hi Chris,

In this pull request, 
1) I fixed the issue in ArcGISRest Preview
2) Add externalLink Preview. This needs an attribute "externalLink" to be contained in Location Field.
3) Add externalDownload for download. The situation of this is we want to point users to a web page which contains the download information and the download link. This needs an attribute externalDownload to be contained in the Location Field.
4) Add the FTP download bean to handle FTP download. The logic is imperfect right now, since I haven't figure out how to set the FTP Serve File Type smartly. Right now for our uses, I hard coded that to Binary, which should apply to most cases. 
Let me know if you need more specification.

BTW, have you merged my ArcGIS WFS download into this branch?