Implemented evaluateScript

Closes #1
Replaces #31
Fixed the implementation of the getScreenshot method
Merge pull request #35 from stof/evaluate_script

Implemented evaluateScript
Merge pull request #36 from stof/fix_screenshot

Fixed the implementation of the getScreenshot method
Fixed the execution of self-executing functions not wrapped in braces
Merge pull request #37 from stof/execute_script

Fixed the execution of self-executing functions not wrapped in braces