Add test for selector
Remove unused fixtures directory

My original intention was to load HTML from files for testing,
but it's simpler to set it using strings instead.
Add tests for each()
Add test for html()
Add tests for text()
Add tests for css()
Make the default target for the build one which works
Expand attr() test

Additional tests to cover:

*   removing pseudo attributes
*   getting the value of an attribute from a single element
*   getting the value of an attribute from a collection
Add tests for removeAttr()
Add tests for removeProp()
Use jsdom rather than domino

domino doesn't support the full DOM API; in particular,
the "elements" property on form elements is missing.

jsdom does a better job of supporting the full DOM API, but
requires an older version of node than I was using. So I changed
the node engine requirements in package.json and .travis.yml
to ensure that tests are run with the correct node version.
Fix removeClass tests to work with jsdom

In domino, $(selector).attr('attr') returns undefined if
an attribute is not defined. In jsdom, the same method call
returns null (which matches the browser more closely).

Fix the tests so they pass with jsdom.
Add tests for serialize()
Add tests for clone()
Add tests for replaceClass()

While writing this test, also added a chai helper to
enable stack traces for failing tests.
Use chai helper throughout test suite

This enables stack traces on all tests if they fail.
Add tests for appendTo()
Use sinon for spies and stubs
Add initial tests for getCssMatrix()