Changed namespace for the shared code between Market Data Providers.
Changed some names to be more appropriate.
Added support for automatic currency conversions in Yahoo! Finance.

As Yahoo! Finance stores data in USD even for european (and other
markets) stock markets, in case a market different than empty or
USD is specified by the user automatically fetch the target
currency for the same time period and use it to convert the
fetched values.
Fixed tests to show correct output on nunit.

Expected value is the first value in Assert.AreEqual(),
while effective value is the second.
Style fixes.