Allow to load tasks from a dir in ENV

You can now specify a directory in ENV['ANVIL_TASKS_DIR'] and it will
load the tasks on it.
Added basic samples
Added missing test
Allow to have multiple dirs on env variable
Merge pull request #21 from anvil-src/samples

Samples and tasks from env variable
Use activesupport 4.0 since 4.1 fails the tests
Merge pull request #24 from anvil-src/fix-activesupport

Use activesupport 4.0 since 4.1 fails the tests
Merge branch 'master' into fix-no-git

Conflicts:
	anvil-core.gemspec