Support of emacs tables.

Tomas,

I've added support of emacs-like tables. Here is the list of tests that I've used. It also shows possible syntax.

    | Markdown | Less      | Pretty                             |
    |----------+:---------:+-----------------------------------:|
    | *Still*  | `renders` | **nicely**                         |
    | 1        | 2         | [Try F# 3.0](http://tryfsharp.org) |

    Markdown|Less|Pretty
    |-|:-:|-:
    *Still*|`renders`|**nicely**
    1|2|[Try F# 3.0](http://tryfsharp.org)|

    |---|:-:|--:|
    *Still*|`renders`|**nicely**
    1|2|[Try F# 3.0](http://tryfsharp.org)

    fruit| price
    -----|-----:
    apple|2.05
    pear|1.37
    orange|3.09

    +--------------------+--------------------+--------------------+
    |    begin           |Code on `C`:        |**Bold**, _italic_, |
    |      writeln('A'); |                    |`code` and          |
    |    end.            |    int main() {    |[link](http://url). |
    |                    |      printf("A\n");|                    |
    |Code on `Pascal`    |      return 0;     |Second paragraph.   |
    |with comments.      |    }               |                    |
    +--------------------+--------------------+--------------------+
    |Quote by Donald     |Nested tables:      |List:               |
    |Knuth:              |                    |                    |
    |                    ||first|second|      |1. First item       |
    |> Beware of bugs    |-:|:-               |2. Second item      |
    |> in the above code;|key|value           |   * unordered list |
    |> I have only proved|                    |3. Third item       |
    |> it correct, not   |:-|-:               |                    |
    |> tried it.         |first|second        |   Some text that   |
    |                    |key|value           |   should be aligned|
    +--------------------+--------------------+--------------------+

    +----------+----------+
    |fruit     |price     |
    +==========+==========+
    |apple     |2.05      |
    +----------+----------+
    |pear      |1.37      |
    +----------+----------+

    +:---------+---------:+
    |fruit     |price     |
    +==========+==========+
    |apple     |2.05      |
    +----------+----------+
    |pear      |1.37      |
    +----------+----------+

I've also refactored code (a little bit) that works with pipe tables and fixed one bug. 

The next feature I miss in FSharp.Markdown is mathematics. Hope I will have time to work on it in the nearest future.