Remove rewrite rule for PNG preview from .htaccess

The .htaccess file has an additional rule to map requests for
PNG preview files to the application PHP script. Eliminate this
rule. Instead, let the URLs to request PNG preview files be the
application home page URL with the hash of the post preview as a
query parameter appended to the home page URL.
Display newlines in error messages in the preview

When an error occurs while converting a post to PNG preview,
the newlines in the error messages are displayed as \n in the
error message. As a result, the newlines are missing from the
error message displayed in the PNG image.

To reproduce the issue, disable JavaScript, create a new post
with the the following content only.

    \foo

Hit the preview button. The following error message would be
displayed.

    pandoc: Error producing PDF from TeX source.\n! Undefined
    control sequence.\nl.46 \foo\n

We want to display the following instead.

    pandoc: Error producing PDF from TeX source.
    ! Undefined control sequence.
    l.46 \foo

Fix this error by joining the output/error lines obtained from
the execution of the pandoc/convert command with newline
character instead of the two byte string '\n'.
Add year 2012 to copyright notice

The copyright notice displays only the year 2013. Display
2012-2013 instead. Also, move the copyright notice to a separate
MathB\View method, so that it can be reused while writing custom
footers in custom views.
Make static preview an optional feature

There is no way to opt out of the static preview feature. It
means that the administrator hosting this application either
needs to install the packages required for static preview, or
end up with error logs in the Apache error log file. When a user
uses the application from a browser with JavaScript disabled, if
the application doesn't succeed in creating a static preview
with the help of packages like pandoc, texlive and imagemagick,
then it would log errors in the Apache error log file.

Make this feature optional, so that if these packages are not
installed, the administrator doesn't have to deal with errors
related to static preview in the logs. This feature would now be
disabled by default, and enabled if required by the
administrator.

Update README.md with information about static preview and the
steps to enable static preview.