Fix/encode

`markdownToHTML` did not care encoding, so in some environments `knit2html` is not available.
Even worse, if windows user has utf-8 encoded .md file, he/she need to convert it into native.enc by hand.
This fix includes:

- `markdownToHTML` accepts `encoding` option.
- If `markdownToHTML` function is called from `knit2html`, it detect the encoding in `knit2html`
- output of `markdownToHTML` is always utf-8 (in the case of file output), in accordance with the meta-tag of HTML.

Please find the inline comments for more technical details.

The updated version has been tested with success under 

- OS X Snow Leopard, locale = "ja_JP.UTF8"
    - encoding: utf8 and CP932
- Windows 7, locale = "Japanese_Japan.CP932"
    - encoding: utf8 and CP932

I'm very very very happy if this fix will be merged and published on CRAN soon.

kohske