THRIFT-2859 JSON generator: output complete descriptors
Client: JSON
Patch: Stig Bakken <stig@zedge.net>

This closes #290

- add --gen json:merge option, and disable merging by default
- output complete descriptors
- add schema for JSON generator
- indent output
'make style' applied
THRIFT-2859: compilation fix, C++11 removed from JSON generator
Increase readability and maintainability in Go generator