(TK-73) Add option to serve symlinks

Add an option to serve symbolic links when serving static
content. This option defaults to false and can be specified
with `add-context-handler` or when specifying static content
in a trapperkeeper config file.
(TK-73) Change symlink option to follow-links

Change symlink option from serve-links to follow-links.
Merge pull request #75 from fpringvaldsen/TK-73-symlinks

(TK-73) Add option to serve symlinks