Added a Progress bar while google map renders

[AkshatSharda]

react-native-google-places is a great work, it works perfectly for me.
When i integrated it in my project i observed that it sometimes takes time to load the maps, when my ram usage is high it take around 4-5 sec to load the map as a result by the time map load it fell like the application hanged. So i have added ProgressDialog showing "Loading maps..." as a text.

[tolu360]

Thanks for your contributions here - I would leave users to doing this in the Javascript side and according to their tastes. Also note that there's a new beta release, please see 198