Merge pull request #171 from CartoDB/great_circle_generator

Adding function to produce a great circle between two points.

doc/CDB_GreatCircle.md

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+Based on Paul Ramsey's [blog post](http://blog.cartodb.com/jets-and-datelines/).
+#### Using the function
+
+Creates a great circle line.
+
+```sql
+SELECT CDB_GreatCircle(start_point, end_point) FROM table_name
+-- Results a line reprsenting the great circle between the two points
+```
+
+#### Arguments
+
+CDB_GreatCircle(start_point, end_point)
+
+* **start_point** ST_Point indicating the start of the line.
+* **end_point** ST_point indicating the end of the line.