Export/import Application Plan limits, pricing rules and features

[eguzki]

A new command application-plan is introduced to export/import application plan artefacts from/to a YAML file.

Export

NAME
    export - export application plan

USAGE
    3scale application-plan export [opts] <remote> <service_id_or_system_name> <plan_system_name>

DESCRIPTION
    Export application plan, including limits, pricing rules and features

OPTIONS
    -f --file=<value>             Write to file instead of stdout

Import

NAME
    import - import application plan

USAGE
    3scale application-plan import [opts] <remote>
    <service_id_or_system_name>

DESCRIPTION
    Import application plan, including limits, pricing rules and features

OPTIONS
    -f --file=<value>             Read from file instead of stdin
    -p --plan-system-name=<value>      Override application plan system name

Fixes #94
Fixes #65

[eguzki]

Feature ready to review @miguelsorianod