Modify altimetry logic, if there is no AGL/AMSL data in the airspace
file, assume that it is AMSL, as that is what ICAO/ATC would do.
forms/club: Added Optional() validator to website field

The URL may also be empty if e.g. just the name is changed.
Merge pull request #178 from shadowoneau/lj-altdefault

Modify altimetry logic in import script
Merge pull request #181 from Turbo87/optional-url

forms/club: Added Optional() validator to website field
assets: Updated bootstrap to v3.0.1
templates: Disabled IE compatibility modes
templates: Use external Respond.js polyfill on IE8 and below

.. as suggested by bootstrap.
assets: Converted respond.js to bundle
Merge pull request #182 from Turbo87/bootstrap

Updated Bootstrap library
Assets: use local copy of ubuntu font
Merge pull request #183 from TobiasLohner/font

Assets: use local copy of ubuntu font