Asynchronous DOT layout in web worker

• The layout work is performed in a web worker to avoid unresponsiveness
• Callbacks and event dispatching to cope with the asynchronicity introduced
• New on() method to register event listeners
• Queuing system to handle application requests while busy
• The transition method accepts a transition factory function for customized transitions and easier animation smoothness and synchronization from an application
• New logEvents() method for debugging and for tuning transition delay and duration

NOTE 1: Although the API in itself is backwards compatible, the behavior is somewhat different because of the asynchronous behavior of web workers. The dot() and renderDot() methods now return "immediately", while the graph layout is being performed in the background. This will for some use cases require adding event listeners to one of the new events. See the README and the example directory for guidance.

NOTE 2: When loading the viz.js script, the script type shall be "javascript/worker", e.g.:
<script src="https://unpkg.com/[email protected]/viz.js" type="javascript/worker"></script>

Added support for growing edges to nodes with URL attribute

Fixes #5

Corrected animated growth of entering edges on Chrome and Safari

Fixes #2

v0.1.0

Animated growth of entering edges.
A demo can be seen here

v0.0.8

Same as v0.0.7, but published to npm without a lot of garbage files included by mistake.

v0.0.7

• Changed d3.graphviz to take selector arg instead of selection

v0.0.4

Updated the README