merge getData and getMCFData

Hi.

We no need anymore use the various functions for core or mcf report API. Methods `getData` and `getMCFData` merged.

Example results:

```r
> ga$getData(id, metrics = "ga:sessions,ga:bounces", dimensions = "ga:browser,ga:city", max = 3)
Only pulling 12 observations of 2297 total (set batch = TRUE to get all observations)
          browser       city sessions bounces
1 Android Browser  (not set)       20      11
2 Android Browser     Almaty        2       2
3 Android Browser Almetyevsk        4       2
> ga$getData(id, metrics = "mcf:totalConversions,mcf:totalConversionValue", dimensions = "mcf:source,mcf:keyword", max = 3)
Only pulling 3 observations of 5019 total (set batch = TRUE to get all observations)
          source   keyword totalConversions totalConversionValue
1       (direct) (not set)             7576                    0
2 173.194.71.113 (not set)                2                    0
3   192.168.32.1 (not set)                1                    0
```

Regards,
Artem Klevtsov