Ignore empty title attributes

Fixes #199.

The fix is not complete though as some tests still fail. This is because conversion is not reversible for empty title attributes:
```
$ kramdown -ihtml -okramdown <<< '<p><img src="a" alt="b" title="" /></p>'
![b](a)
```
but
```
$ kramdown -ikramdown -ohtml <<< '![b](a)'
<p><img src="a" alt="b" /></p>
```
and therefore `assert_equal` fails.

Any suggestions? Should I avoid these tests?