CB-5017 accept proxy config for plugman

[xliyong]

plugman does not honor proxy configuration. take the same apporach as npm itself and use "request" module to handle proxy config.

this commit also introduces changes to dependencies:

• request
• tar
• tar.gz

[novocaine]

+1, seriously inconvenient that it doesn't work

[Urbiwanus]

Any progress on this issue ?

[Pijuli]

This is the only workarround I've found in 7 days of battles behind a proxy. When will it be added to the main package?

[agrieve]

Merged. Thanks!

Please close the pull request.