Add a short alias for each command from propel 1.6 command names

Refs #200

I took the alias from Propel 1.6 as @fzaninotto suggested (https://github.com/propelorm/Propel/blob/master/generator/build.xml#L82)

The help becomes like this:
```
Propel version 2.0.0-dev

Usage:
  [options] command [arguments]

Options:
  --help           -h Display this help message.
  --quiet          -q Do not output any message.
  --verbose        -v Increase verbosity of messages.
  --version        -V Display this application version.
  --ansi              Force ANSI output.
  --no-ansi           Disable ANSI output.
  --no-interaction -n Do not ask any interactive question.

Available commands:
  build                Build the model classes based on Propel XML schemas
  convert-conf         Transform the XML configuration to PHP code leveraging the ServiceContainer
  diff                 Generate diff classes
  down                 Execute migrations down
  graphviz             Generate Graphviz files (.dot)
  help                 Displays help for a command
  insert-sql           Insert SQL statements
  list                 Lists commands
  migrate              Execute all pending migrations
  reverse              Reverse-engineer a XML schema file based on given database
  sql                  Build SQL files
  status               Get migration status
  up                   Execute migrations up
config
  config:convert-xml   Transform the XML configuration to PHP code leveraging the ServiceContainer
database
  database:reverse     Reverse-engineer a XML schema file based on given database
graphviz
  graphviz:generate    Generate Graphviz files (.dot)
migration
  migration:diff       Generate diff classes
  migration:down       Execute migrations down
  migration:migrate    Execute all pending migrations
  migration:status     Get migration status
  migration:up         Execute migrations up
model
  model:build          Build the model classes based on Propel XML schemas
sql
  sql:build            Build SQL files
  sql:insert           Insert SQL statements
test
  test:prepare         Prepare the Propel test suite by building fixtures
```