Add implementation of StringPath class

`StringPath` class is something similar to Ruby's `Pathname` class,
but does not involve any IO operations. `StringPath` objects require
passing string representation of path, and array of paths that
represents universe to constructor to be intantiated.

lib/gitlab/string_path.rb

+module Gitlab
+  ## 
+  # Class that represents a path to a file or directory
+  #
+  # This is IO-operations safe class, that does similar job to 
+  # Ruby's Pathname but without the risk of accessing filesystem.
+  #
+  #
+  class StringPath
+    def initialize(path, universe)
+      @path = path
+      @universe = universe
+    end
+
+    def absolute?
+      @path.start_with?('/')
+    end
+
+    def relative?
+      !absolute?
+    end
+
+    def directory?
+      @path.end_with?('/')
+    end
+
+    def file?
+      !directory?
+    end
+
+    def to_s
+      @path
+    end
+  end
+end

spec/lib/gitlab/string_path_spec.rb

+require 'spec_helper'
+
+describe Gitlab::StringPath do
+  let(:universe) do
+    ['path/dir_1/',
+     'path/dir_1/file_1',
+     'path/second_dir',
+     'path/second_dir/dir_3/file_2',
+     'path/second_dir/dir_3/file_3',
+     'another_file',
+     '/file/with/absolute_path']
+  end
+
+  describe '/file/with/absolute_path' do
+    subject { described_class.new('/file/with/absolute_path', universe) }
+
+    it { is_expected.to be_absolute } 
+    it { is_expected.to_not be_relative }
+    it { is_expected.to be_file }
+  end
+end