Adding additional targets. Initially I had only the simplest setup.
Refactored base build.xml file to reduce the need for overrides:

- Added required.packages property to version.properties file with
  sequential list of required packages
- Changed to version.managed in version.properties file for referencing
  the main package instead of the hard coded version.<NAMESPACE>
- Renamed deployManagedUAT to deployManagedBeta
- Renamed updateDependentPackages to updateRequiredPackages
- Changed getPackageVersions to use required.packages list instead of
  hard coded list of package version requirements
- Changed updateDependentPackages to use required.packages list instead
  of hard coded list of package version requirements
- Added preUpdateRequiredPackages target designed to be overriden to
  implement logic for uninstalling packages in complex extended package
use cases
- Added updateMetaXml method
Changed ant-salesforce.xml to cumulusci.xml and fixed reference in main
build.xml
Getting CumulusCI path from property.
Adding updateMetaXml target to parent build file.
Merge branch 'feature/02-centralize-build-scripts' of https://github.com/SalesforceFoundation/CumulusCI into feature/02-centralize-build-scripts
Merge pull request #6 from SalesforceFoundation/feature/02-centralize-build-scripts

Adding additional targets. Initially I had only the simplest setup.