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)

[prjemian]

travis-ci reports success but the logs say different:

======================================================================
ERROR: test_newfile_exists (test_filewriter.Test_SpecWriterCallback)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/test_filewriter.py", line 146, in test_newfile_exists
    specwriter.newfile(filename=testfile)
  File "tests/../apstools/filewriters.py", line 531, in newfile
    from spec2nexus.spec import SpecDataFile
ModuleNotFoundError: No module named 'spec2nexus'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "tests/test_filewriter.py", line 151, in test_newfile_exists
    self.assertFalse(raised, "file exists")
UnboundLocalError: local variable 'raised' referenced before assignment