set a default build dir and auto-create it

The most common case does not need to know or care where the build
directory is, because they're using `statocles daemon` to test it.

Additionally, cleaning the directory should always be as simple as `rm
-rf <BUILDDIR>`, which means we should automatically create the build
dir every time.

Fixes #233
Fixes #234

lib/Statocles/Site.pm

@@ -116,8 +116,22 @@ also the store the C<daemon> command reads to serve the site.
 has build_store => (
     is => 'ro',
     isa => Store,
-    required => 1,
-    coerce => Store->coercion,
+    default => sub {
+        my $path = '.statocles-build';
+        if ( !-d $path ) {
+            # Automatically make the build directory
+            mkdir $path;
+        }
+        return Store->coercion->( $path );
+    },
+    coerce => sub {
+        my ( $arg ) = @_;
+        if ( !ref $arg && !-d $arg ) {
+            # Automatically make the build directory
+            mkdir $arg;
+        }
+        return Store->coercion->( $arg );
+    },
 );
 
 =attr deploy

t/site.t

@@ -9,16 +9,39 @@ use Mojo::URL;
 my $SHARE_DIR = path( __DIR__, 'share' );
 
 subtest 'constructor' => sub {
+    my $cwd = cwd;
+    my $tmp = tempdir;
+    chdir $tmp;
+    mkdir '.statocles-build';
+
+    my %required = (
+        deploy => '.',
+    );
+
     test_constructor(
         'Statocles::Site',
-        required => {
-            build_store => '.',
-            deploy => '.',
-        },
+        required => \%required,
         default => {
             theme => Statocles::Theme->new( store => '::default' ),
+            build_store => Statocles::Store::File->new( path => '.statocles-build' ),
         },
     );
+
+    chdir $cwd;
+
+    subtest 'build dir gets created automatically' => sub {
+        my $tmp = tempdir;
+        chdir $tmp;
+
+        lives_ok { Statocles::Site->new( %required ) };
+        ok -d '.statocles-build', 'directory was created';
+
+        lives_ok { Statocles::Site->new( build_store => 'builddir', %required ) };
+        ok -d 'builddir', 'directory was created';
+
+        chdir $cwd;
+    };
+
 };
 
 subtest 'site writes application' => sub {