An initial pass at exposign the asynchronous nature of librdkafka up into Ruby

This should be enough to get some initial review of the API going. Hooking up a
Hermann::Result class to the Hermann::Producer in Ruby is in this commit but
actually driving the librdkafka reactor underneath is not yet hooked up.

Fixes #11
Fixes #15
Fixes #18
Avoid double-ocpying and a buffer overrun when pushing a Ruby string into Kafka

This approach to shuffling the message buffer into librdkafka relies on librdkafka to safely
copy the buffer for it's own uses from the Ruby string's C pointer.

This has been tested with buffers 2x the size of the SIGSEGV mentioned in #21

Fixes #21
Fixes #13
Merge pull request #20 from rtyler/issues/11-producer-feedback

An initial pass at exposing the async nature of librdkafka upwards
Merge pull request #22 from rtyler/issues/21-buffer-overrun

Avoid double-ocpying and a buffer overrun when pushing a Ruby string
Add a simple script to validate big message behavior

References #21
Properly pass the block to Consumer#consume into the C layer
Pass a pointer to a Hermann::Result into the message delivery callback

Provided that /something/ is ticking the librdkafka reactor underneath, the
Hermann::Result will be updated with the message delivery status when that's
completed.

What's still missing is producer level error calbacks

Fixes #11
Fixes #15
Fixes #17
Tie Hermann::Result#value to the underlying reactor to bring values up to Ruby

This ensures that we're getting async values out of librdkafka into the calling
Ruby thread. Currently errors aren't being brought up properly, but we're getting there

Example:

    [14:47:30] tyler:Hermann git:(issues/11-producer-feedback*) $ pry -I lib -r 'hermann/producer'
    [1] pry(main)> p = Hermann::Producer.new('topic', 'kafka0.REDACTED.com:6667')
    => #<Hermann::Producer:0x00000803b3b450
    @brokers="kafka0.REDACTED.com:6667",
    @children=[],
    @internal=#<Hermann::Lib::Producer:0x00000803b3b3d8>,
    @topic="topic">
    [2] pry(main)> r = p.push('hello world!')
    => #<Hermann::Result:0x00000803b8cb20
    @producer=
    #<Hermann::Producer:0x00000803b3b450
    @brokers="kafka0.REDACTED.com:6667",
    @children=[#<Hermann::Result:0x00000803b8cb20 ...>],
    @internal=#<Hermann::Lib::Producer:0x00000803b3b3d8>,
    @topic="topic">,
    @reason=nil,
    @state=:unfulfilled,
    @value=nil>
    [3] pry(main)> r.state
    => :unfulfilled
    [4] pry(main)> r.value
    ticking rdkafka reactor
    ticked
    => "hello world!"
    [5] pry(main)> r.state
    => :fulfilled
    [6] pry(main)> r.rejected?
    => false
    [7] pry(main)>
    [14:47:56] tyler:Hermann git:(issues/11-producer-feedback*) $

Fixes #11
Merge pull request #23 from rtyler/issues/11-producer-feedback

Tie Hermann::Result to the librdkafka reactor
Use RSTRING_LEN instead of _LENINT which isn't present on 1.8.7

Fixes #30
Remove exit(..) calls and replace by raising a runtime exception to Ruby.