encapsulating sender methods

…so it is possible to only send a webmention/pingback.
check if link header is an array

if you have more than one link header $headers['Link'] is an array.
Merge pull request #2 from pfefferle/master

methods to only sendping backs/webmentions and a link-header fix
look for webmention endpoint before pingback
refactors MentionClient to expose two static methods for just sending the payload. Added examples to the readme
add test for when there are two Link headers
update example in readme