first sketch of a web optimized cog

[vincentsarago]

closes #10

[vincentsarago]

There is still something odd with how I get the output transform to align with the mercator grid.
The top X value is good but all other are a bit shifted (top Y, bottom x, bottom y)

internal tile: (2.0819091796875, 45.03471477868859, 2.083282470703124, 45.035687477951846)
mercator tile: (2.0819091796875, 45.03568524531632, 2.083282470703125, 45.03665569548622)

tried to show the diff here : https://gist.github.com/vincentsarago/6521bbaf07d2c5128c98e1fd7b1d4275
taking the for 512/512px block bounds and the top left z18 tile of a file

[vincentsarago]

Alright, there are too many unrelated changes in this PR which make it complexe to merge/debug. I'm going to close this one and split the changes into different PR, if this is ok with you @perrygeo @sgillies