Added versioner class to deal with semver strings

by @jondeandres and @franciscoj
Protect some versioner methods
Documented Versioner class
Set semantic's version on the gemspec
gem_bump task to bump the version
Add tests for gem bump task

In addition to that, I've added a logger to the Anvil namespace. Since
one of the purposes of anvil is to automatice tasks, it makes sense that
it writes its log to a log file. For now it will write to the stdout.
Namespace gem tasks
Task to push gems
Commit and tag after bumping
Added missing test for new gem version task
Now we can build anvil without rake :smile: