Link abbrev feature

Hi,

As requested in issue #52, I now manage link abbreviations.
Tell me if the logic is ok for you.

Here is an Org example:

    #+LINK: example http://example.com/something
    #+LINK: example2 http://example.com/something2

    [[example][This an example]]
    [[example2][This is the second example]]
    [[http://example.com/standard][Standard example]]
this is the end