Use node-liftoff to improve CLI mode.

[Liftoff](https://github.com/tkellen/node-liftoff/) was initially written for Grunt, and improves the CLI experience :
* Finds `Gearfile` or `Gearfile.js` from any project subfolder. It just walks up the directory tree until it finds the configuration file.
* Better global vs local integration : a globally installed Gear.js instance first searches if there is a locally installed instance. Local instances always have the priority.
* `--cwd` and `--Gearfile` (with an uppercase G - this could be improved) command-line options allow multiple gearfiles, and to run gear from anywhere.

The minimist argument parser also gives me a few cool ideas, like :
* a `--version` option which would display all gear/gear-lib/external tasks version numbers
* allow multiple sections in a single Gearfile.