addLink: bugfix and QoL improvements

All changes in pdf.PdfFileWriter.addLink:
Added documentation to make use a bit clearer.
Fixed an issue where /P was mapped to a repr'd IndirectObject instead of
a proper indirect reference.
Added support for a border-styling parameter.
Allowed rect to be a string (as before), or a RectangleObject, or a
list, to make use more intuitive.
Bugfix for dash patterns

Fixed a bug where a border with a dash pattern (e.g. [0, 0, 1, [2, 3]])
would fail to write.
str-hacks cleanup

Found the `Str` util; replaced the version_info hack in the new addLink
functionality with the proper `Str` usage.
Merge pull request #104 from HenryKeiter/master

addLink: Bugfix and Quality-of-Life Improvements