Make DirectedMultigraph forbid loops.
Fix test.

Remove the loop from v1 to v1 and add an edge from v2 to v1.
Fix test:Use assertEquals(expected, actual, delta)

To compare floating-point numbers, we need to provide a delta. This
fixes a failing build using Java 7.
Add DirectedWeightedPseudograph.
Update copyright year.
Merge pull request #31 from agouge/feature/directed-weighted-pseudograph

Add DirectedWeightedPseudograph
Merge pull request #30 from agouge/fix/java-7-build

Quick Fix (one line)
Merge pull request #29 from agouge/fix/directed-multigraph

Fix DirectedMultigraph
Credits for last merge.