remove hard dependency on fog

Load fog only if needed, and emit a helpful message if it's missing.

This removes a ton of dependencies, which makes the default installation
easier for newcomers.

lib/riemann/dash.rb

@@ -2,7 +2,5 @@ require 'rubygems'
 require 'sinatra/base'
 require 'riemann/dash/version'
 require 'riemann/dash/browser_config'
-require 'riemann/dash/browser_config/file'
-require 'riemann/dash/browser_config/s3'
 require 'riemann/dash/config'
 require 'riemann/dash/app'

lib/riemann/dash/config.rb

@@ -68,8 +68,14 @@ class Riemann::Dash::Config
     uri = URI.parse(ws_config_file)
     backend = case uri.scheme
             when "s3"
-              Riemann::Dash::BrowserConfig::S3.new(uri.host, uri.path.sub(/^\//, ''), store[:s3_config])
+              begin
+                require 'riemann/dash/browser_config/s3'
+                Riemann::Dash::BrowserConfig::S3.new(uri.host, uri.path.sub(/^\//, ''), store[:s3_config])
+              rescue LoadError
+                raise Exception.new 'Fog library required to save to S3. Run: "gem install fog"'
+              end
             when nil, "file"
+              require 'riemann/dash/browser_config/file'
               Riemann::Dash::BrowserConfig::File.new(uri.path)
             else
               raise Exception.new "Unknown backend for #{ws_config_file}"

riemann-dash.gemspec

@@ -21,7 +21,6 @@ Gem::Specification.new do |gem|
   gem.add_dependency 'sass', '>= 3.1.14'
   gem.add_dependency 'thin', '~> 1.6.2'
   gem.add_dependency 'multi_json', '1.3.6'
-  gem.add_dependency 'fog'
   gem.files         = `git ls-files`.split($/)
   gem.executables   = gem.files.grep(%r{^bin/}).map{ |f| File.basename(f) }
   gem.test_files    = gem.files.grep(%r{^(test|spec|features)/})