Add new payswarm markup and parsing code.

- Support for new tags:
  BEGIN/END_PAYSWARM_PAID_CONTENT tags are used to mark the
    beginning and end of content that must be paid for to be shown.
    If an unpaid content tag is used after the beginning tag, it
    will terminate the content as if the end paid content tag
    were there.
  BEGIN/END_PAYSWARM_UNPAID_ONLY_CONTENT tags are used to mark
    the beginning and end of content that will only be shown when
    a post has not been paid for. This is different from the
    default text that always shows regardless of payment. If a
    begin paid content tag is used before an end unpaid content
    tag is found, it will terminate the content as if the end
    unpaid content tag was there.
  PAYSWARM_ACCESS_BUTTON tags are used to place payswarm access
    buttons in the content. These can be used anywhere in a post
    and they have an optional parameter that can follow the tag
    (and a space). The parameter can be a simple text string that
    will be displayed next to the button and before the price. If
    this tag is not used and paid content is in the article, a
    default access button and text will be displayed at the end
    of the article.