Json dump

This branch adds 'json' config option to cursor, to create JSON output.
Output items appear for example as:
```
"key0" : 123
```
and values as:
```
"value0" : "string part"
"value1" : 234"
```
That is, columns, separated by newlines.  If columns are named, those names are used instead of key0, value0, etc.

Can be used with index cursors as well as table cursors.
New ```wt jsondump``` utility that uses json cursors.  refs #740.