Basic JSON support

Here is a patch for basic JSON support. It adds only those features:
  * Add "json" data type, that is binary-compatible with text.
  * Syntax checking on text to JSON conversion.
  * json_pretty() -- print JSON tree with indentation.

We have "JSON datatype (WIP) 01" item:
  https://commitfest.postgresql.org/action/patch_view?id=351
but it seems too complex for me to apply all of the feature
at once, especially JSON-Path support. So, I'd like to submit
only basic and minimal JSON datatype support at first.

The most different point of my patch is that JSON parser is
reimplemented with flex and bison to make maintenance easier.

Note that the JSON parser accept top-level scalar values
intensionally, because of requirement when we add functions
to extract values from JSON array/object. For example,
  CREATE FUNCTION json_to_array(json) RETURNS json[]
might return naked scalar values in the result array.

--
Itagaki Takahiro