Add existsSync module that works on node 0.6

This change makes all the tests pass in `node` `0.6` - which also means that any dependents of it won't automatically be broken in `v0.6` :-)
assuming this is the only reason tests are not passing on 0.6.. do you mind moving this shim to `common.js`? not sure it deserves a whole file of its own :) (so far I've been using one file per ShellJS command, except for common.js)