Wrong/missing handling of 'g:org_tags_column' value

[doak]

According to orgmode's builtin documentation the value of g:org_tags_column shall be handled like this:

"The column to which tags should be indented in a headline. If this number is positive, it specifies the column. If it is negative, it means that the tags should be flushright to that column. For example, -80 works well for a normal 80 character screen. When 0, place tags directly after headline text, with only one space in between."

Currently vim-orgmode handles positive values like it should handle negative values, negative values are not handled at all. Furthermore the value 0 is separated with two spaces (instead of with only one).

To be fair, the documentation of orgmode itself is not clear about the behaviour.

I am using current master (v0.6.0-28-gc6cd668).

This issue could be loosely related to #31.