update missing capability

capability:myaddinstance (moodle 2.6+)
fix missing 'myaddinstance'

and correct one mistype
Check constant existence before defining it

Reference issue #23
Merge pull request #24 from Mihailoff/patch-1

Check constant existence before defining it
Merge pull request #21 from lengyelke/CR_23_STABLE

Missing capability (add instance my page)
Fixed plot graph with series names with html entities