Multiple symbols for MOEX data connector

[Mottl]

Now MOEX data connector supports multiple symbols in constructor:

import pandas_datareader.data as web

df = web.DataReader(['USD000UTSTOM', 'SBER'], 'moex',
                    start='2017-07-01', end='2017-07-31')

closes #562

[Mottl]

@bashtage, could you look at travis? Errors aren't related to this commit.

[khazamov]

@Mottl thanks Dmitry! That was a very quick implementation :)