Adds environment boolean checks

Amber currently ships with 3 different environments: development, test
and production.

As a developer I would like to sometimes run code conditionally
depending on the environment that the application is.

```crystal
if Amber::Env.development?
  # ... code here ...
end

```

This PR addresses the issue by creating a Amber::Env module that
contains 3 module methods:

Amber::Env.development?
Amber::Env.test?
Amber::Env.production?

With the changes in this PR a developer should be able to call the
appropriate method to check for the desired environment.

spec/amber/server/env_spec.cr

@@ -0,0 +1,17 @@
+require "../../../spec_helper"
+
+describe Amber::Env do
+  {% for env in Amber::Env::ENVIRONMENTS %}
+    describe ".{{env.id}}?" do
+      it "returns true when the environment is {{env.id}}" do
+        Amber::Settings.env = {{env}}
+        Amber::Env.{{env.id}}?.should eq true
+      end
+
+      it "returns false when the environment does not match" do
+        Amber::Settings.env = "invalid environment"
+        Amber::Env.{{env.id}}?.should eq false
+      end
+    end
+  {% end %}
+end

src/amber/server/env.cr

@@ -0,0 +1,9 @@
+module Amber
+  module ENV
+    {% for env in Settings::ENVIRONMENTS %}
+      def self.{{env.id}}?
+        Settings.env.not_nil!.downcase == {{env.downcase.id.stringify}}
+      end
+    {% end %}
+  end
+end

src/amber/server/settings.cr

@@ -23,7 +23,8 @@ module Amber
     class_property redis_url = ""
     class_property session : Hash(Symbol, Symbol | String | Int32)
 
-    # loads settings from environment yaml
+    # Loads environment yml settings from the current AMBER_ENV environment variable
+    # and defaults to development environment
     {{ run("./environment.cr") }}
   end
 end