Convert B<> to ** instead of __ to allow nesting with I<>

### The problem

If POD has nested bold/italicized elements, the Markdown that gets generated is misinterpreted by many  Markdown parsers.

For example, the following:

    I<B<BOLD + ITALICS> JUST ITALICS>

Gets converted to:

    ___BOLD + ITALICS__ JUST ITALICS_

Which is misinterpreted GitHub's Markdown parser as:

___BOLD + ITALICS__ JUST ITALICS_

After making a couple changes, it now gets converted to the Markdown string:

    _**BOLD + ITALICS** JUST ITALICS_

which is properly parsed:

_**BOLD + ITALICS** JUST ITALICS_

### My solution

I made the change previously suggested on line 715 of `lib/Pod/Markdown.pm` to use `**` instead of `__`.

I also had to change the `_escape_paragraph_markdown` subroutine, because it was causing problems with bolded words at the beginning of a line (the first asterisks was escaped). This isn't an issue though, since asterisks are properly escaped by the `_escape_inline_markdown` subroutine (which doesn't cause the same problem). (Perhaps `B<>` gets converted after inline escaping and before paragraph escaping perhaps?)

Also, the module still passes all 151 tests for me.