SECF() implementation

[bverh]

Closes #49 - all tests added / passed

>>> import pdblp
>>> con = pdblp.BCon(port=8194, timeout=5000)
>>> con.start()
>>> con.secf("IBM")
                        ticker                                        description
0                IBM US EQUITY        International Business Machines Corp (U.S.)
1                IBM UN EQUITY    International Business Machines Corp (New York)
2                IBM LN EQUITY      International Business Machines Corp (London)
3  IBM US 01/17/20 C145 EQUITY              January 20 Calls on IBM US Strike 145
4                IBM GR EQUITY     International Business Machines Corp (Germany)
5                IBM FP EQUITY  International Business Machines Corp (Euronext...
6  IBM US 10/19/18 C150 EQUITY              October 18 Calls on IBM US Strike 150
7                      IBM PFD               International Business Machines Corp
8                IBM CN EQUITY      International Business Machines Corp (Canada)

Returns a pd.DataFrame to be consistent with other functions.

[matthewgilbert]

Other than those minor things all looks good, thanks for the pull request.