Add support for footnotes

This changes does two things:

1) It adds a custom var to hold the class for wikimedia tables. I made the default, 'wikitable' which seems to be supported and produces a nice table (see screen shot). I could see making the default 'nil' if you want.

2) This diff adds basic support for footnotes for this ticket:  https://github.com/craftkiller/orgmode-mediawiki/issues/5

Here is a test ORG file I used, its markup output after export and a screen shot of what the exported mark up looks like on a wiki page.

Thanks,
--greg.

## Sample Org File
<pre>
* Main Section

  Hi, this  is going to test  footnotes[fn:footnote]. That last one  was a named
  footnote, this one[fn:1] is a numbered footnote.

** sub section
   Now, here comes an inline footnote[fn:: defined inline] and the next one will
   be the alternate version of the inlined footnote[fn:alternate: An alternate style linline].

   That is all. Please move along.

#+CAPTION: [tester]{A table showing stuff.}
#+LABEL: tab:value-tables
#+ATTR_LATEX: :environment longtable
# #+NAME:value-tables   
|---------------+-------+-------+--------+-------------|
| Website Title | Price | Value | Orders | Total Value |
|---------------+-------+-------+--------+-------------|
| One Year      |    99 |  1188 |  28000 |    33264000 |
| test one      |    69 |  1400 |  19000 |    26600000 |
| test two      |    69 |  1400 |  18800 |    26320000 |

* Footnotes

[fn:footnote] this one works.

[fn:1] and this one too.

</pre>

## Exported markup from example org file.
<pre>
= Main Section =
Hi, this  is going to test  footnotes[1]. That last one  was a named
footnote, this one[2] is a numbered footnote.

== sub section ==
Now, here comes an inline footnote[3] and the next one will
be the alternate version of the inlined footnote[4].

That is all. Please move along.


{| class=wikitable






|-

|-
|Website Title
|Price
|Value
|Orders
|Total Value

|-

|-
|One Year
|99
|1188
|28000
|33264000

|-
|test one
|69
|1400
|19000
|26600000

|-
|test two
|69
|1400
|18800
|26320000

|}

----
=== Footnotes ===

[1] this one works.

[2] and this one too.

[3] defined inline

[4] An alternate style linline

</pre>



## Screen Shot of markup

![screen shot 2013-12-24 at 9 23 07 am](https://f.cloud.github.com/assets/1823356/1806839/f5c47504-6cc0-11e3-862c-f2f97441a3ba.png)