Pgraster

Closes #191
Includes testcase
Requires development version of grainstore (see https://github.com/CartoDB/grainstore/pull/95)
Apart from comment, which would now be invalid, the new code would just trust internal caller (not user) to either want a grid or a tile. The code should be slightly faster due to the === equality check and a single test (but that's minor). It'll be useful when/if MVT will be supported in output.