src: fix multi-base class ObjectWrap::Unwrap<T>()

Fix pointer unwrapping when T is a class with more than one base class.

Before this commit, the wrapped void* pointer was cast directly to T*
without going through ObjectWrap* first, possibly leading to a class
instance pointer that points to the wrong vtable.

This change required some cleanup in various files; some classes
used private rather than public inheritance, others didn't derive
from ObjectWrap at all...

Fixes #6188.
process: Add exitCode property

This allows one to set a specific status code, while still letting the
process exit gracefully once all async operations are completed.
process: Use exit code 8 consistently

This should always be used in the case of an uncaughtException
process: Fix regression from a5dba82

Respect numeric string args to process.exit(code)
process: use exit 1 for uncaughtException

Since it is Unix tradition to use exit code 1 for general-purpose script
bail-out, and the way of doing that in Node is to throw an exception and
not catch it, it makes the most sense to exit with 1 when an exception
goes uncaught.

Move the `Invalid Argument` exit to 9, so that it's something specific,
and clear that it's a node internal error.

Also, document the exit codes that we use.
npm: upgrade to 1.3.11
Merge remote-tracking branch 'ry/v0.10'
test: move slow tests to test/pummel/

Slowness being somewhat subjective but determined by running the
test suite a few times and picking off everything that consistently
clocks in at 2 seconds or more.

Honorable mention for simple/test-tls-server-large-request, it often
runs for 10 (!) seconds or more.
buffer: optimize common encoding cases

String#toLowerCase() is incredibly slow and was costing a 15-30%
performance hit for Buffers less than 1KB. Now instead it'll attempt to
find the correct encoding directly from the passed encoding, only then
afterwards it'll lowercase.

The optimization for not passing any encoding at all is still at the top
of the method.

At most this may add 10% performance hit for passing a mixed case
encoding.
contextify: dealloc only after global and sandbox

Functions created using: `vm.runInNewContext('(function() { })')` will
reference only `proxy_global_` object and not `sandbox_`. Thus in case,
where there're no references to sandbox (such as in example above),
`ContextifyContext` will be destroyed and use-after-free might happen.
http: don't pass the request options to Agent

The `options` that were being passed in before here are specific to a
single request, which kinda defeats the purpose of using an Agent in the
first place.

On a worse note, these `options` have not yet been "processed" by the
`http.ClientRequest` class, so if `port: null` is set (like it is as the
result of a `url.parse()` call), then they take preference over the
processed values since the agent's "options" get mixed in last in the
`createSocket()` function.

Fixes #6197.
Fixes #6199.
Closes #6231.
contextify: fix ContextifyContext leak

Apparently, context->Global() won't be destroyed if the context itself
isn't marked as weak and independent.

Also, the weakness flag should be cleared once the weak callback is
executed, otherwise we'll get crashes in Debug builds.

fix #6115 and #6201