Added an in operator to std.json.JSONValue. Tested when const/non-const
Squashed commit of the following:

commit 19f81880aa206939eb643e400d7b0a02c5f06475
Author: Mark Isaacson <markisaa@fb.com>
Date:   Wed May 21 15:46:49 2014 -0700

    Bugfix. Sorry about lots of rapid commits for tiny changes

commit 653306579699c6e1ae911ba8d72ba9d6c720dd5b
Author: Mark Isaacson <markisaa@fb.com>
Date:   Wed May 21 15:45:26 2014 -0700

    Partial specialization instead of template constraint for detecting operator

commit 000d9be503a5b06753980c5abfcac8d216f6bba3
Author: Mark Isaacson <markisaa@fb.com>
Date:   Wed May 21 15:44:13 2014 -0700

    Removed redundant auto

commit 0021a5024a5bf42138ba18814b85b5f499649fda
Author: Mark Isaacson <markisaa@fb.com>
Date:   Wed May 21 15:42:20 2014 -0700

    Return pointer not bool