The resolver module implements a path resolving algorithm that
leverages node's own require.

This has a couple benefits:
* more likely to continue working as expected when features are
introduced to npm/node
* reduction of code duplication (npm/node already implements the module
resolution we are interested in for this project)
* allow the use of NODE_PATH variable to control module directory

Additionally, `resolver` leverages `Package` and `Import` constructors
which implement some of the same logic previously found on the `find`
function in `lib/importer.js`. The benefit of creating these
constructors will obvious when unit tests are introduced for this
project.

`Package`: implements logic related to retrieving package data, such as
getting the package.json, and getting the sass 'main' file for a given
package.

`Import`: implements logic to make sense of a sass import.

This allows the tests to work consistently

Also improved style in tests

Because string expansion is not compatible with all shells

This helps some of the async code easier to read