Switch from RJSONIO to jsonlite
Update spec tests and add data spec test
Update backslash handling
Cleaner handling of var names

This commit separates the conversion of an expression (a name or call)
to a string, and the escaping of '.' for safe vega vars. This makes it
possible to use correct titles for axes legends when the data column
has a '.' in it.
Update tests
Merge branch 'jsonfix'