Project specific environment variable

Added a feature to set some environment variables during creation of project, specific to the project.

Use case:

Without branching out, to test out features in the project which are toggled by use of environment variables.

In this case we cannot use the goldberg_config.rb to change the command that is executed, for instance let it be:

    rake spec

Instead we can set the environment variable while creating the project:

    bin/goldberg add https://github.com/user/repo repo_ci --branch master --scm git --env foo=bar foobar=zoo 

So then while running the build, the command which will be called is:

    foo=bar foobar=zoo rake spec

If the config.environment_string is also set to say "bar=foo" then it will be called as:

    foo=bar foobar=zoo bar=foo rake spec