calculate:

Added sketch of `calculate` app.
urls:

Added url mapping for `calculate` app.
static:

Reworked the leaflet-based page styling to have cleaner sizing
and rescaling of the map and sidebar.
templates/index.html:

Refactored the index template. Nothing has really change, I just moved
some divs around.
templates/oq_base.html:

Added menu link for Calculate page.
templates/calculate.html:

Added `New` and `Browse...` calculation buttons.
static/css/oqplatform_leaflet.css:

Added some margin styling to ribbon controls.
templates/oq_base.html:

Transposed the order of the oq-body-content and oq-body-sidebar divs.

In normal cases, the sidebar should come first since it is located to
the far left.
templates/leaflet_base.html:

Added a layer-tree div to the base leaflet template.
static/css/oqplatform_leaflet.css:

Added styling for the sidebar and layer tree.
templates/calculate.html:

Removed a TODO and renamed the Browse... button to Open....
static/js/base_leaflet_app.js:

Implemented a more concise way of setting the height of the main
page components (the map and sidebar).
Factored out the common base map url

It's now defined 1 time in base_leaflet_app.js.
Merge pull request #92 from larsbutler/calculate-tab

Calculate tab

[r=matley] [f=1233255]