Fix traingle => triangle and Mdiamon => Mdiamond typos
Complete the list of edge and node styles

Ref. http://www.graphviz.org/doc/info/attrs.html#k:style
Fix lookup for headport= and tailport=

Without this small fix headport= and tailport= would not be completed.
Also, write a better help text for those options.
Merge pull request #4 from bjornfor/fix-typos

Fix traingle => triangle and Mdiamon => Mdiamond typos
Merge pull request #5 from bjornfor/styles

Complete the list of edge and node styles
Merge pull request #6 from bjornfor/port-fixes

Fix lookup for headport= and tailport=
Update README.md