set proxy from enviroment variables
change setting of proxy base on node.js code
(https://github.com/joyent/node/blob/master/deps/npm/node_modules/npmconf/config-defs.js#L232)
[FIX] add version attribute to callback call in nodist instal method
[TEST] set proxy to env attribute of exec test calls, base on enviroment
       variables proxy settings
Merge pull request #22 from gratex/master

support proxy