allow skipping pending migrations using an environment variable

if PENDING_MIGRATIONS_OK is set, the usual pending migration check will not be
performed. this is useful for automation tools (e.g. bots that do deployments).

do not set this variable for your regular deployments unless you are absolutely
sure what you are getting into.
bump version to 0.0.5
add warning message when PENDING_MIGRATIONS_OK and REVERSE_DEPLOY_OK are set

adding logger.warn statement to make sure users don't miss the fact that environment
variables that caused these checks to be skipped was set.
Merge pull request #21 from PagerDuty/environment-variable-to-skip-migration-check

use an environment variable to skip pending migrations check
handle log directory not being present for DualLogger

not all projects that use pd-cap-recipes (or capistrano) have a log directory
where cap command is executed from.

this is to fix DualLogger initialization by rescuing error that may be raised
due to log directory not being present and assigning nil to file. Logger will
handle output file being passed as nil graciously.
bump version to 0.0.6
Merge pull request #22 from PagerDuty/check-logger-before-logging

handle File.open raising an error and failing
remove assets.rb (deploy:assets:* tasks)

with this commit, following tasks will be removed:
- deploy:assets:cdn_deploy
- deploy:assets:precompile (override for the original capistrano task)

also removing tests associated with this file assets_spec.rb and removing
assets section from README.
bump version to 0.0.7