Print to provided output stream in deprecated 'write-json'

Originally reported by Benjamin Teuber in a GitHub pull request:
https://github.com/clojure/data.json/pull/9
DJSON-7: Omit comma after key/value pairs removed by :value-fn

Signed-off-by: Stuart Sierra <mail@stuartsierra.com>
Update README for release 0.2.2
[maven-release-plugin] prepare release data.json-0.2.2
[maven-release-plugin] prepare for next development iteration
Update to latest pom.contrib parent version
DJSON-11: Fix printing of unnecessary commas with value-fn

Adds state to loop remembering whether any key/value pairs have been
printed before, to distinguish whether to print a comma or not.

Changes test case to use sorted-map, both for determinism in the
order, and to exhibit the bug with the uncorrected version of
write-object.

Signed-off-by: Stuart Sierra <mail@stuartsierra.com>
DJSON-9: Optionally escape special separator characters

Even when :escape-unicode is false, users may want to escape the
Unicode separator charators U+2028 and U+2029, which are valid in pure
JSON but not valid in JavaScript strings.

See http://timelessrepo.com/json-isnt-a-javascript-subset

Original patch contributed by Tim McCormack.
Clarify docstrings for :value-fn

See comments on DJSON-12, especially:
http://dev.clojure.org/jira/browse/DJSON-12?focusedCommentId=31605&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-31605
Remove extraneous whitespace in docstring
Rename 'separators' to 'js-separators'; see DJSON-9
Rename 'separators' to 'js-separators' in tests
Update README for 0.2.3 release
[maven-release-plugin] prepare release data.json-0.2.3
[maven-release-plugin] prepare for next development iteration
DJSON-15: Correct an incorrectly written test

Signed-off-by: Stuart Sierra <mail@stuartsierra.com>
README: Update change log
DJSON-14: handle EOF inside character escape
README: Update change log for DJSON-14
DJSON-13: newline & flush output after pprint

Makes clojure.data.json/pprint behave more like clojure.pprint/pprint,
including the fix for CLJ-987.

clojure.data.json/pprint now flushes the output stream and ensures
that its output ends with a newline. This is a small change in
behavior.