Allow reading and writing Excel files in remote sources

[juarezr]

Today the excel implementation only allows reading and writing Excel files in the local filesystem.
Other formats like avro, csv, json, text already support reading from remote servers/cloud using fsspec package.
This improves the current implementation for allowing opening remote xls and xlsx files when using fsspec.

Changes

1. Changed fromxls() and toxls() to call read_source_from_arg or write_source_from_arg.
2. Changed fromxlsx(), toxlsx() and appendxlsx() to call read_source_from_arg or write_source_from_arg.
3. Added tests for testing remote testing of the xls and xlsx file formats.
4. Added appendxlsx() to init.py making it public
5. Tested with S3 remote source.

Checklist

Checklist for for pull requests including new code and/or changes to existing code...

• Code
  • Includes unit tests
  • New functions have docstrings with examples that can be run with doctest
  • New functions are included in API docs
  • Docstrings include notes for any changes to API or behaviour
  • All changes documented in docs/changes.rst
• Testing
  • (Optional) Tested local against remote servers
  • Travis CI passes (unit tests run under Linux)
  • AppVeyor CI passes (unit tests run under Windows)
  • Unit test coverage has not decreased (see Coveralls)
• Changes
  • Ready to review
  • Ready to merge

[coveralls]

Coverage decreased (-0.1%) to 91.781% when pulling 3293c68 on juarezr:xlsx_remotes into d713928 on petl-developers:master.

[juarezr]

This PR conflicts with #502 because it changes the same lines of code for Excel support.

So I thinking on:

1. merging this PR
2. after this PR is merged rework Allow toxlsx() to add or replace a worksheet. #502
3. test, review and merge the changes in Allow toxlsx() to add or replace a worksheet. #502 reworked

Any suggestions?

[juarezr]

Closing this PR because it's changes were merged in #509.