Add .to.be.a.symlink() support. Closes #3

I haven't written any unit tests for it because I can't work out how to get the test suite to work - it always fails with some rather strange errors:

```
[...]

308:  path_exist.js invalid data (bad value type) should fail when negated assert (with message)

      failed but expected message 'My Message: value: expected 123 to be a string' to equal 'My Message: function (data) {\n      return render.call(this, data, _);\n    }'
         at Assertion.<anonymous> (/home/dave/chai-fs/test/tester.js:34:10)
         at Assertion.ctx.(anonymous function) [as failCall] (/home/dave/chai-fs/node_modules/chai/lib/chai/utils/addMethod.js:40:25)
         at Context.<anonymous> (/home/dave/chai-fs/test/tester.js:113:13)

      -  My Message:    function (data) {\n
        "||||||||||||+++-|-----------------"
      +  My Message: val u

               re turn re  nder.call(t his, data,    _);\n
        "-------|+----|-|++--|--|----|+-----|----|+++-----"
                e:      exp  e  c    te     d     123

                        }
        "|++|++|+|++++++-"
          to be a string

-> failed 308 and passed 86 of 394 tests, left 28 pending (818ms)

Warning: Task "mochaTest:pass" failed. Use --force to continue.
```

Any suggestions?