#64 fix display precision of big numbers

[baumgold]

Rafactor and simplify the table2json method as follows:

• Finite numbers larger than 2^53-1 or smaller than -2^53-1 will be treated as a string and enclosed in double-quotes
• Nothing and Missing will use the repr function to convert to string
• Everything else will use the print function to convert to string
• Use JSON helper methods to generate the JSON string
• Reduce indentation for improved readability

Closes #64

[baumgold]

@pfitzseb - Any comments here? Thanks!