zlib: don't assert on missing dictionary

Handle Z_NEED_DICT errors graciously. Fixes the following assertion:

  node: ../src/node_zlib.cc:161: static void node::ZCtx::Process
  (uv_work_t*): Assertion`ctx->dictionary_ != __null && "Stream
  has no dictionary"' failed.
  Aborted (core dumped)

Fixes #4632.
zlib: don't assert on malformed dictionary

Handle Z_DATA_ERROR errors from inflateSetDictionary() gracefully.
Fixes the following assertion:

  node: ../src/node_zlib.cc:167: static void node::ZCtx::Process
  (uv_work_t*): Assertion `ctx->err_ == 0 && "Failed to set
  dictionary"' failed.
  Aborted (core dumped)

Fixes #4632.
doc: document PYTHON env var in README
blog: Post for v0.9.8
doc: fix typo in crypto Hash.digest() note

Added the word 'has' to the sentence.
Closes #4652.
benchmark: Port http.sh from master
buffer: slow buffer copy compatibility fix

Fix issue where SlowBuffers couldn't be passed as target to Buffer
copy().

Also included checks to see if Argument parameters are defined before
assigning their values. This offered ~3x's performance gain.

Backport of 16bbecc from master branch. Closes #4633.
doc: fix typo in cluster module documentation

Fix a small typo in the cluster.settings documentation.