Added footer to read_html
read_html neglected table footers. Although rare, some sites use the footer for data. First time pull request so hopefully not messed up.

Needs a test.
 Add it in `tests/test_html.py`.

It might be good to have a test that exercises the `<footer>` parsing on a very simple example (maybe even hand written HTML) and then another test with an example of some HTML from the wild that contains `<footer>`s.
 Should probably also test an empty footer like `<footer></footer>`.
 Ok thanks! I was going to add my local test to tests/test_html.py but found an error which meant I hadn't tested this modification properly (and now getting NaNs where there shouldn't be). Will keep working on it, and getting my head around how to use git.
 I was getting `NaN` as my test (and real world example) used `<td>` rather than `<th>` inside `<tfoot></tfoot>` so it wasn't parsing correctly.
 @mjsu 
- pls add a release note documenting this in enhancements in v0.15.2 (reference this issue)
- pls squash

@cpcloud will review after
 Hmm, I think I have messed this up! (still very novice at git/github)
 @mjsu ?
 Yep?
 @mjsu just pinging in u. This needs a rebase / squash and to add those items mentioned above.
 I thought I did it. I wasn't quite sure what I was doing so may have done it wrong?
 see here https://github.com/pydata/pandas/wiki/Using-Git

try

```
git rebase -i origin/master
```

squash to 1 commit

then

```
git push myfork thisbranchname -f
```
 That done it? (slowly getting my head around this stuff, thanks for the help!)
 good.

@cpcloud ?
 @cpcloud pls have a look and merge if ok (you prob have to rebase it)
 @cpcloud ?
 looks good to me, needs a rebase looks like there's some merge conflicts.
 @mjsu thanks!
