test: remove "should" dependency

The test suite is not a suitable place for clever code. With should it's
sometimes difficult to know exactly what is being asserted. For example:

    should.not.exist(err);

The following assertion is unambiguous:

    assert.strictEqual(err, null);
remove .npmignore
Merge pull request #14 from plaid/dc-should-not

test: remove "should" dependency
init: throw if passed an invalid config object
init: avoid mutating config object