Route kernel messages about language_info, execution_state, and execution count into ydoc

[Zsailer]

Fixes #55 and #10

Will update this comment with more info later.

• simplifies the output processor
• makes the kernel message cache more critical (with unit tests)
• uses a switch in the kernel_client to route messages into the doc
• ensures language_info and document execution state is accurate
• ensures execution_state and execution count to be accurate in doc.

TODO

• update execution time in ydoc
• clear messages from the cache in a "smart" wa

[ellisonbg]

Overall this is a great direction. I have left a bunch of minor comments and suggestions below.

[3coins]

@Zsailer
This looks great!. Added some minor suggestions.

[Zsailer]

Closing in favor of #90, which has been ported to the new package name.

Will address comments further over there.