refactor common EARL reporter code into abstract parent class

The test manifest and test result reporters share some common code that
can be put in a parent class. For consistency, also generate JSON-LD
from the test manifest reporter.
Merge pull request #112 from spadgett/master

fix test-manifest profile in pom.xml
Merge pull request #113 from spadgett/earl-reporters

refactor common EARL reporter code into abstract parent class
refactor common EARL reporter code into abstract parent class

The test manifest and test result reporters share some common code that
can be put in a parent class. For consistency, also generate JSON-LD
from the test manifest reporter and use the same namespace prefixes as
turtle.
use common prefixes for turtle and JSON-LD EARL reports