fix flexReport URL

[gnzsnz]

Provide mechanism to overwrite default ULR with an environment variable

Updated docstring

ib_async.flexreport.FlexReport?
Init signature: ib_async.flexreport.FlexReport(token=None, queryId=None, path=None)
Docstring:     
To obtain a token:

* Login to web portal
* Go to Settings
* Click on "Configure Flex Web Service"
* Generate token
Init docstring:
Download a report by giving a valid ``token`` and ``queryId``,
or load from file by giving a valid ``path``.

To overwrite default URL, set env variable ``IB_FLEXREPORT_URL``.
File:           ~/Documents/dev/ib_async_fixes/ib_async/flexreport.py
Type:           type
Subclasses:     

[mattsta]

Thanks! Looking to get these additional fixes added to the cleanup branch and released this weekend.