Adds DAT_PORT environmental variable to set dat serve port
usage: Document clone arguments (optional directory)

'dat clone' accepts an optional directory argument since 4fad9f4 (dat
clone works more like git clone now, 2014-04-10).  Mention that in the
usage docs.

I'm not very familiar with the range of JavaScript libraries, but if
there is something similar to Python's argparse module that lets you
declare your interface and then automatically constructs the parser
and usage docs, that would make it easier to keep the parser and docs
in sync.
usage: Avoid "no method 'codespan'" error

With Markdown-style `code` markup, I was getting:

  $ dat help
  /.../dat/node_modules/ansimd/node_modules/marked/lib/marked.js:1209
      throw e;
            ^
  TypeError: Object #<Object> has no method 'codespan'
  Please report this to https://github.com/chjj/marked.
      at InlineLexer.output (/.../dat/node_modules/ansimd/node_modules/marked/lib/marked.js:660:28)
      at Parser.tok (/.../dat/node_modules/ansimd/node_modules/marked/lib/marked.js:1064:50)
      at Parser.parse (/.../dat/node_modules/ansimd/node_modules/marked/lib/marked.js:924:17)
      at Function.Parser.parse (/.../dat/node_modules/ansimd/node_modules/marked/lib/marked.js:911:17)
      at marked (/.../dat/node_modules/ansimd/node_modules/marked/lib/marked.js:1201:19)
      at render (/.../dat/node_modules/ansimd/index.js:21:13)
      at Object.dat.help (/.../dat/lib/commands.js:166:15)
      at ready (/.../dat/cli.js:60:35)
      at /.../dat/index.js:105:32
      at /.../dat/lib/commands.js:654:25

Avoid that by using a code block for the syntax declaration.  It looks
like the current release of marked (which we pull in via ansimd) is
indeed missing code spans [1].

[1]: https://github.com/chjj/marked/blob/v0.3.2/README.md#inline-level-renderer-methods
Merge pull request #124 from wking/clone-docs

cli.js: Document clone arguments (optional directory)
refactor primary key options into document.js, use in put and write-stream
log IP address
new progress log
fix ref error in push progress
bump dat-replicator
use conflict event in write stream instead of error event
upgrade tape
return 409 for conflicts
binary encode/decoding support in schema
update changes stream .data to .value
default writeStream to objectMode, get rid of 'objects' setting
use dat-editor prebuilt for home page
remove old require
basic transformations compiler for #125
unref subprocesses so it doesnt hang dat
read and normalize dat.json