Variable interpolation support

[kamermans]

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%;"