Check db/migrate/ instead of schema.rb

This avoids the problems with schema_format :sql. Fixes #57, #59 and #74.
bin/mina exit_status -> exitstatus
Update doc on #invoke helper

I lost 1hour on this trying to call a task multiple times.. because I didn't notice the __options__ line of the doc. So I thought a documentation change would be nice for others :)
Make tmp directory if it doesn't exist

In mina-deploy/mina#225, gabskoro noted that the tmp folder might not
exist in all Rails projects. So, touching restart.txt won't actually do
anything.

This creates the tmp directory, in case it doesn't exist, with mkdir -p.
Unfortunately, there isn't a flag for touch which forces intermediate
directory creation, so we've got to use a separate command.
Merge pull request #229 from dvdhsu/master

Make tmp directory if it doesn't exist
Update Readme.md
Allow the foreman export format to be configurable.

* Added the foreman_format setting.
Updated deploy.rb template to use shared_path

Use `shared_path` in case the user configures it.
Merge pull request #235 from postmodern/patch-1

Updated deploy.rb template to use shared_path
Update HISTORY.md
Merge pull request #232 from postmodern/foreman_export_format

Allow the foreman export format to be configurable.
Update HISTORY.md
Merge pull request #226 from popox/patch-1

Update doc on #invoke helper
Merge pull request #212 from Zhomart/fix198

fix: bin/mina exit_status -> exitstatus
Update HISTORY.md
Support pretty_system on ruby 1.8.7
Add foreman_location config
Add foreman_sudo config
Ruby 1.8.7 doesn't support empty symbols
Merge pull request #239 from PChambino/foreman_location

Add foreman_location and foreman_sudo configs