Refactoring: remove unused variables and update tests.

:wave: Hello &yet! 

After going through the code, I found some parts of the base that could be refactored.

Issue 1: After adding `unused: true` to the `.jshintrc`, there are defined but unused variables.

```sh
ampersand-state.js
  line 78   col 28  'options' is defined but never used.
  line 101  col 13  'triggers' is defined but never used.
  line 298  col 13  'type' is defined but never used.
  line 680  col 9   'prop' is defined but never used.
  line 680  col 15  'item' is defined but never used.
  line 753  col 9   'toString' is defined but never used.

test/basics.js
  line 130   col 51  'value' is defined but never used.
  line 130   col 44  'model' is defined but never used.
  line 520   col 9   'lastError' is defined but never used.
  line 531   col 42  'error' is defined but never used.
  line 531   col 35  'model' is defined but never used.

test/full.js
  line 244   col 9   'one' is defined but never used.
  line 601   col 44  'newVal' is defined but never used.
  line 601   col 36  'oldVal' is defined but never used.
  line 800   col 9   'thing' is defined but never used.
  line 1133  col 9   'sub2' is defined but never used.
  line 1261  col 9   'emptyattrs' is defined but never used.
  line 1262  col 9   'undefinedattrs' is defined but never used.
  line 1257  col 39  'opts' is defined but never used.
  line 1332  col 9   'model' is defined but never used.
```

Issue 2: Because I have set `unused: true`, the tests pass but `jshint` complains because there are variables which are defined but are not used. Within the test, the respective values assigned to the variables are the ones being examined under `testling`.

Issue 3: The test for Issue #1791 is [missing an assertion](https://github.com/francismakes/ampersand-state/compare/AmpersandJS:master...francismakes:master#diff-d52eeda9ba89df3f626cf88a2d2a1879L1330).

Issue 4: Some tests are missing a `t.plan(#)`.

Issue 5: Some test titles are wrapped in double quotes instead of single quotes (not that it really matters...)


This PR:
1. Includes an updated `.jshintrc` with `unused` set to true;
2. Use instances directly without variable assignments (see [here](https://github.com/francismakes/ampersand-state/compare/AmpersandJS:master...francismakes:master#diff-d52eeda9ba89df3f626cf88a2d2a1879R836) and [here](https://github.com/francismakes/ampersand-state/compare/AmpersandJS:master...francismakes:master#diff-d52eeda9ba89df3f626cf88a2d2a1879R1304));
3. Includes `t.plan(#)` where missing; and
4. Updates any and all test names wrapped in double quotes to single quotes.

I didn't see a contributing guide, but am inclined to make the needed revisions where necessary. Please let me know if I'm missing anything too. Thanks in advance!

`options` argument is used here https://github.com/francismakes/ampersand-state/blob/master/ampersand-state.js#L13 and can be useful when overriding default parse method with some specific options while state initialization.