Currently, `sources=[]` by default, which will become "" once the
list is `joined` to form a csv string. The tiingo api appears to
interpret the empty string as "don't match any sources" instead of
"match all sources". This commit fixes the problem by setting
`sources` to `None` if the list is empty.
Adds unit test for empty new sources list.
Updates HISTORY.rst with note about bug fix.
Add unit tests for new methods.
Update fixtures to support the etf and mutual fund tests.
Change filename inside of zip to supported_tickers.csv
Add second entry into supported_tickers.csv
Added myself to authors.
