Simplify things by storing integer values only as int64_t's internally, and

Simplify things by storing integer values only as int64_t's internally, and
 omit the range check during parsing since we already have the checks when
 accessing the value. There is no longer a json_type_int64, only json_type_int.
Fix some problems with parsing 0 and -0 values, and add a couple of tests.
Fix some minor compile issues on HPUX environments.

git-svn-id: http://svn.metaparadigm.com/svn/json-c/trunk@60 327403b1-1117-474d-bef2-5cb71233fd97