Refactor common shell functions out.
Allow the project_root task to take arguments.

Previously, the project_root task would only tell you the project root
for the subproject you were in.  Now you can pass in another project
name after a colon to get the root of any subproject.

For example, `buildr project_root:common` can be run anywhere to
give you the root directory of the common project.
Merge pull request #651 from candlepin/awood/common_bash

Refactor common shell functions out.