Execute now inherits from MyriaPage to get access to base_template_vars.
(Maybe consider unifying MyriaHandler and MyriaPage?)

classes Datasets and Execute now pull from base_template_vars to have a single point of control over host name.

Add a method getRESThost to MyriaPage that controls the host name for REST calls.  Now set to <GAE>/rest

change get_connection_string to use this REST host

Datasets is now taking a long time to load -- perhaps firing one REST call per dataset?  Maybe need to do all this work on the server.