support for download experiment from the exchange

just uses git basically but also interfaces with the API to record # of downloads, etc...  command is
`psiturk-install` and instructions about its use are provided on each of the individual experiment exchange pages (e.g., http://dev.psiturk.org/ee/HW3XfPCTUjPDdtE87aHRHi)
adding support for custom models
Merge branch 'refs/heads/expxchange_commandline_tool' into dev