WIP/ENH: allow read_excel to accept URLs (GH6809)
closes #6809

This borrows the mechanisms already used in read_csv to enable reading of Excel objects via URLs.

Not sure what tests are appropriate, and I'm not certain what the various decorators do, so I tried to mirror the read_csv tests as best I could.

looks good.... @jorisvandenbossche @cpcloud comments?
 (Removed vestigial `import xlrd`.)
 looks good
 thanks @dsm054 glad to have you back on the wagon!
 https://travis-ci.org/pydata/pandas/jobs/30344018

any idea why the network decorator didn't catch this? (maybe need to add this exact error message to the error list)...
