Variable interpolation support

See #52 

Support is built into the core now, via `Builder::interpolate()`, and I've added support to the `Mysql` and `Env` plugins.

Similar to the Env plugin, the following vars are interploated (with example values):

    %PHPCI%   =1
    %PHPCI_COMMIT%   =52e9aba3fae11c3b1db11f4d36b72c2862ea3cf0
    %PHPCI_PROJECT%   =3
    %PHPCI_BUILD%   =61
    %PHPCI_PROJECT_TITLE%   =MyApp
    %PHPCI_BUILD_PATH%   =/opt/PHPCI/build/project3-build61/

I am using it with the MySQL plugin.  It looks a little nasty, but it works as expected:

    setup:
        mysql:
            - "DROP DATABASE IF EXISTS phpci_project%PHPCI_PROJECT%_build%PHPCI_BUILD%;"
            - "CREATE DATABASE phpci_project%PHPCI_PROJECT%_build%PHPCI_BUILD%;"
            - "GRANT ALL PRIVILEGES ON phpci_project%PHPCI_PROJECT%_build%PHPCI_BUILD%.* TO phpunit_tester@'localhost' IDENTIFIED BY 'phpunit';"
            - import: { file: "protected/data/database.sql", database: phpci_project%PHPCI_PROJECT%_build%PHPCI_BUILD% }
    complete:
        mysql:
            - "DROP DATABASE IF EXISTS phpci_project%PHPCI_PROJECT%_build%PHPCI_BUILD%;"