Add block support for Workbook#initialize. This closes #31

[artembaikuzin]

Add block yield inside Workbook#initialize. Usage:

WriteExcel.new('ruby.xls') do |book|
  worksheet = book.add_worksheet
  worksheet.write('A1', 'Hi Excel!')
end

[artembaikuzin]

Build has failed on ruby 1.9.3:

$ bundle --version
Bundler version 1.7.6
$ gem --version
2.6.8
12.26s$ bundle install --jobs=3 --retry=3
Fetching gem metadata from https://rubygems.org/..............
Resolving dependencies...
Installing docile 1.1.5
Installing rake 12.0.0
Installing power_assert 1.0.1
Installing simplecov-html 0.10.0
NoMethodError: undefined method `spec' for nil:NilClass
Installing json 2.0.3
An error occurred while installing writeexcel (1.0.6), and Bundler cannot
continue.
Make sure that `gem install writeexcel -v '1.0.6'` succeeds before bundling.
The command "eval bundle install --jobs=3 --retry=3 " failed. Retrying, 2 of 3.

Maybe there is some problem with bundler version 1.7.6?