Fixed the 'already initialized constant TAG' warning by converting to class level methods

Thanks for the gem!  I noticed some warnings about redeclared constants, so I switched them to static methods.

BEFORE CHANGE:

Andrews-MacBook-Pro:docx aforward$ bundle exec irb
irb(main):001:0> require "docx"; Docx::Document
/Users/aforward/tp/projects/docx/lib/docx/elements/bookmark.rb:9: warning: already initialized constant TAG
/Users/aforward/tp/projects/docx/lib/docx/elements/text.rb:6: warning: already initialized constant TAG
/Users/aforward/tp/projects/docx/lib/docx/containers/text_run.rb:16: warning: already initialized constant TAG
/Users/aforward/tp/projects/docx/lib/docx/containers/paragraph.rb:11: warning: already initialized constant TAG
=> Docx::Document

AFTER CHANGE:

Andrews-MacBook-Pro:docx aforward$ bundle exec irb
irb(main):001:0> require "docx"; Docx::Document
=> Docx::Document