make sure shared/pids and shared/log get created for non-Rails apps (closes #5)

No defaults are specified in `shared_paths` unless you load Vlad's Rails
recipe. We need the shared/log and shared/pids directory for Unicorn
itself, though. This change adds { 'pids' => 'tmp/pids', 'log' => 'log'
} to the shared_paths hash if those keys aren't already set. Note that
this is safe because `Rake::RemoteTask.set_defaults` initializes
`shared_paths` to an empty hash.

--HG--
extra : amend_source : 77670b79992e41aee68d7962a27b1084e25b178b
factor out writing command to start unicorn

--HG--
extra : histedit_source : 918116fa6c50f48a4e34b9f4e360d4c26429e05c
add simple option to use bundle exec

--HG--
extra : amend_source : 14702a4436df69cf40becd32b5c2694acfd7dfb6
extra : histedit_source : a1af4a26a91af8577c595f4eda9e15bcfa26545b
fix shell-quoting of sudo commands

The quoting introduced in a50bac41dd43 suffered from a backslash-quoting
issue of its own.
simplify setting unicorn_env

Use Rake::RemoteTask's fetch method with a default instead of catching the
exception it throws.
quote config file path
prepare to release 2.3.0

--HG--
extra : rebase_source : e2d502c12e8da8c758d13e4387f7cbed6349cbf2
extra : amend_source : 2e3940088eff17e240bc085edc551ad51f57c420
README.txt: formatting fixes
only pass -E if unicorn_env is set

Before actually releasing this option, I'm pretty sure I want to have it
disabled by default. But I also want it to be easy to enable the
sensible default behavior (fall back to rails_env if present, otherwise
'production').
update Rails configuration docs

--HG--
extra : amend_source : e0f2e227f2dae142bfd49d83d2ee235fdf34465d
docs: last updates before releasing 2.3.0
tagging v2.3.0 for release