[fix] Don't leak globals when used as a function

When used as in `README` (called as `require('back')()`), the module was
leaking globals because it used `this` when not being in the right
context.

This caused problems when using more than one `back` instances created
like that.
Merge pull request #1 from mmalecki/fix-function

[fix] Don't leak globals when used as a function