support configurable postgresql data directory

[rvalyi]

rationale:

You can use Devstep with embedded Postgresql like you would use sqlite in a Rails project: your databases are persisted in a project directory without cluttering a database server. You find back the projects databases easily and you ca easily trash the database directory whenever you want some clean up.

For that, we need to be able to have the Postgresql data directory for instance in /workspace/.db not hardcoded as it is currently in the Postgresql configuration.

A working example of that https://gist.githubusercontent.com/rvalyi/91706bbb7478dd46d70a/raw/56bbcce1fcbbb7a8ee3c4f95089ec7defe6ec6c5/devsetp-pg
Now, ideally it shouldn't be hardcoded like in my patch. Having it as an environment variable may be great.

[fgrehm]

This is an easy one to handle, it would be a matter of accepting an env var over here with the current path as the default like we do for other env vars and you could easily set it up from your .devsteprc either globally or for a specific project.

If that works for you I'll try to get it in place for the 0.1.0 tomorrow or a following 0.1.1 (maybe at some point next week)

[rvalyi]

yes, exactly that will be great thanks

[fgrehm]

Scheduling for 0.1.1, unfortunately I won't have time to implement and properly test this today =/

[fgrehm]

Done on GH-73