Fix multiple embedded resources (see blongden/hal#14).

When a HAL response contains multiple embedded resources of the same
'rel', they are parsed at once.

This fails, as it is an array with separate valid HAL resources in it.
This is fixed by checking if the embeds are an array, and if so, adding
the resources one-by-one.

This is related to blongden/hal#14.

Signed-off-by: Jacob Kiers <jacob@alphacomm.nl>
Merge pull request #22 from jacobkiers/bugfix/fix-multiple-embedded-resources

Fix multiple embedded resources (see blongden/hal#14).
Fix for #20 - only uses 'value' as element value if it's the only peice
of data
Fix no links generated when self URL is null
Check embedded resources have a self url set
Add phpunit as dev dependency