Travis and Coveralls integration

We'll be adding TravisCI and Coveralls integration to a Boost library. Let's take a Boost.LexicalCast as an example. At the end we'll get ​something like this.

Travis requires some access permissions to the repository that are usually provided by Boostorg Admins. However bothering Admins is actually not required:

• go to the Boost project page (in our example it would be ​https://github.com/boostorg/lexical_cast)
• press the "Fork" button at the top right corner. Now you have a copy of the boost project in your account.
• clone the original boostorg repo or use an existing clone. Change directory to the clone folder (cd boost_maintain/boost/libs/lexical_cast/)
• add our forked repo as another remote host for default push: git remote set-url --add origin git@github.com:<your_login>/lexical_cast.git. If everything is right, you'll see something like this if you execute git remote -v:

  origin	git@github.com:boostorg/lexical_cast.git (fetch)
  origin	git@github.com:boostorg/lexical_cast.git (push)
  origin	git@github.com:<your_login>/lexical_cast.git (push)
  

• log in into ​Travis
• go to the ​https://travis-ci.org/profile/ and enable builds for the forked project.
• log into ​Coverals.
• go to the ​https://coveralls.io/repos/new and add a forked repo.

• now add the .travis.yml file, tune the README.md and commit changes:

  cp ../variant/.travis.yml ./                       # Copying .travis.yml from Boost.Variant
  git add .travis.yml                                # Adding .travis.yml for next commit
  
  cp ../variant/README.md ./                         # Copying README.md from Boost.Variant
  sed -i 's/variant/lexical_cast/g' README.md        # Replacing `variant` with `lexical_cast`
  sed -i 's/apolukhin/<your_login>/g' README.md      # Replace `apolukhin` login name wuth your own login name
  gedit README.md                                    # ... editing README.md in editor
  git add README.md                                  # Adding README.md for next commit
  
  git commit -m "Travis and Coveralls integration"   # Committing changes to GIT
  git push                                           # push changes to boostorg and forked repo using one command
  

That's it! If everything is done right, you'll see the build process going on TravisCI and after ~20 minutes results will be visible in README.md view on github.