make general method to export JSON datasets

[prjemian]

In #136, code was developed to export datasets from the USAXS instrument into a JSON text file. Generalize that code and contribute to apstools.utils.

[prjemian]

ZIP the export file.

[prjemian]

Could be a command-line application, also.

[prjemian]

travis-ci reports it cannot import pyRestTable
It is downloaded and installed but conda list does not show it. Something must be off in the travis config.

[prjemian]

skipping the command-line entry-point (for now)