Add more commands and options

Thanks for a great grunt plugin!

In order to incorporate it into our workflow, I had to add a few more commands and options:

#### New Options:
1. defaultSchemaName
2. defaultsFile
3. logLevel

#### New Commands:
1. rollback
2. rollbackCount
3. tag

I also added support for the grunt CLI flag `--no-write`. If this flag is specified, then the liquibase command will not actually be executed. This was very useful for me when it came to testing my changes. 

I've also documented the new options in the README, and converted tabs to two spaces.

If you disagree with any of these changes, I'm happy re-work them, or put them in a separate pull request.

Thanks!