change coastlines to OSMD data sources, fix #121

HDM and osm-bright both use the same sources for coastline and water (but with different layer names), share the same styling for water at zoom levels below 10, and have the same issues (great lakes does not render before zoom level 7 and the files are no longer updated and are outdated)

My pull requests for HDM and in osm-bright fixes both issues.

Before my pull request: 
for zoom: >=0 <6 
HDM & bright both use: http://mapbox-geodata.s3.amazonaws.com/natural-earth-1.3.0/physical/10m-land.zip (natural earth 1.3.0)
previously known in natural earth as NE_10M_LAND.zip  
now known in natural earth: as "ne_10m_land_scale_rank.zip
(ref: https://github.com/nvkelso/natural-earth-vector/blob/master/CHANGELOG)
HDM layer name: land-low
BRight layer's name "land"

at zoom >=6 <10: 
HDM & bright both use: http://tilemill-data.s3.amazonaws.com/osm/shoreline_300.zip
hdm layer name is land-mid
bright layer name is shoreline_300

at >=10: 
HDM & bright use: http://tilemill-data.s3.amazonaws.com/osm/coastline-good.zip"
HDM layer name: is land-high
bright layer name is: processed_p 

Instead, openstreetmap-carto simply uses 2 files: 

for zoom <10: 
http://data.openstreetmapdata.com/simplified-land-polygons-complete-3857.zip
layer named: world 

The simplified land polygon file includes nice detail even though it's simplified, for example, the Great Lakes at Zoom 7:
![osm-cartoz7](https://f.cloud.github.com/assets/955351/1646810/a8909692-5928-11e3-8e4a-5dc502b0d6a4.png)


for >10: 
openstreetmap-carto uses: 
http://data.openstreetmapdata.com/land-polygons-split-3857.zip
(layer named: coast-poly)

I don't see any initial disadvantage to completely dropping the Natural earth file and moving to both of these files 
as indicated in my pull requests. 

I do have a couple questions and concerns: 
1. HDM and bright have 2 layers (water_gen1 and water_gen0) https://github.com/mapbox/osm-bright/blob/master/osm-bright/osm-bright.osm2pgsql.mml#L163 for creating water at low zoom levels: 

https://github.com/hotosm/HDM-CartoCSS/blob/master/base.mss#L202 

In both layers, water objects whose area is larger than 1,000 or 10,000 are selected and there may be some instances of where water is being drawn in these layers over the existing map-background, water. 


For what it's worth, openstreetmap-carto also likely has instances where it draws water over map-background's water since it starts rendering water from the sql query at z6: https://github.com/gravitystorm/openstreetmap-carto/blob/master/water.mss#L28

As long as it doesn't hamper performance, I think it's inevitable that there will be instances of water drawn over water.

Aside, this means the Great Lakes aren't rendered as water at 2 opportunities: 
1. The old natural earth SHP used at zoom >=0 <6 in bright/HDM treats the great lakes as land; 
2. For some tagging reason (@pnorman may know perhaps), the water_Gen0 query does not select the Great Lakes. 

Other concerns: 

2. The openstreetmapdata.com files are usually updated daily. This can be troublesome for lower bandwidth users as tilemill will automatically download the newer file in the background. @yohanboniface: Do we want to use a script as mentioned in ? 


3. As [@ajashton mentioned](https://github.com/mapbox/osm-bright/issues/71#issuecomment-20706311), the zip files from openstreetmapdata are not indexed in shapefile. 
Will this hamper speed?