Entity decoding in Text tokens

Now the decode function for each will decode HTML entities:

``` haskell
> Text.HTML.TagStream.ByteString.decode "foo bar &quot;mu&lt; zot &hello;"
Right [Text "foo bar \"mu< zot "]
> Text.HTML.TagStream.Text.decode "foo bar &quot;mu&lt; zot &hello;"
Right [Text "foo bar \"mu< zot "]
```

The conduit (thanks to @snoyberg) also works:

``` haskell
> CL.sourceList (T.chunksOf 1 "&gt;") C.$= T.tokenStream C.$$ CL.consume
[Text ">"]
```

In the spirit of tagstream-conduit's liberal parsing, invalid or unknown entities are skipped:

``` haskell
> Text.HTML.TagStream.Text.decode "&foo; &bar &quot;mu&lt; zot &hello;"
Right [Text "  \"mu< zot "]
```

A test case has been added.