Added :use_local_env variable

Normally capistrano-local-precompile launches tasks with the production defined environment.
The problem is that normally you should not allow remote connections to your production database (so rails application can't start locally with the production environment)
To avoid the problem I made changes to make capistrano-local-precompile to add '-local' to original's environment (if :use_local_env is set to true).
This requires some work (you have to define everything for the new environment) but it's worth to avoid security problems.

I changes also specs to reflect changes in the code.

I hope you will adopt changes,
Simone