Updated command for custom executors

- app.cmd is no longer a string
Add v0.7.0 label to new API endpoints
prevent mutable state from leaking out of TaskTracker
Tests for TaskBuilder with args/executor
Minor enhancements to Docker doc.
Merge pull request #489 from mesosphere/v7-rest-api-label

Add v0.7.0 label to new API endpoints
Merge pull request #497 from mesosphere/docker-doc

Minor enhancements to Docker doc.
Create doc for upgrading from 0.6.x to 0.7.0
Update to use site.baseurl
Word Fixes per @ssk2 review.

Update Gemfile and README.md to more accurately reflect what is need to run the docs site.
Merge pull request #498 from BenWhitehead/0.7.0-upgrade-docs

Create doc for upgrading from 0.6.x to 0.7.0
Update docs with mesos compatibility.
Merge pull request #499 from BenWhitehead/upgrade-docs-update

Update docs with mesos compatibility.
Updated groups error message for readability
Merge pull request #503 from mesosphere/groups-error-msg

Updated groups error message for readability
Fix missing comma
Merge pull request #504 from BenWhitehead/docs-comma

Fix missing comma
Group and app names in groups validation error.
Added app deployments to REST docs
Merge pull request #507 from mesosphere/app-deployments-docs

Added app deployments to REST docs