Highlight Links, bug fixing and testing

Closed issues:

• How can I get onMouseOverLink event? #25
• Click one node but another one moves #41

Merged pull requests:

• Fix/tests coverage #44 (danielcaldas)
• Fix/on drag node handler #42 (danielcaldas)
• Feature/on mouse over and out link #40 (danielcaldas)
• Set proper defaults for Graph component config #39 (danielcaldas)
• Fix semantics mouse over methods in Graph component #38 (danielcaldas)