set LINEMAN_ENV based on CLI command.

This is only set internally and cannot be overridden by the user for
the spec, spec-ci, or build commands
Upgrade fetcher to support append/prepend
Support programmatic async calls.
Grunt task for lineman fetch CLI action.

Usage: 
$ lineman grunt fetch:facebook-connect-api-all:google-analytics:twitter-widgets-api
0.33.0