Create Pull Request should set destination appropriately instead of using master

When you create a pull request from the sourcetree client, it redirects you bitbucket and starts creating a pull request there. Unfortunately, it always sets the destination to master, which is rarely the case. Sometimes people will forget to change that destination, which could lead to people accidentally merging changes into master without going through development branches. I think it should be smarter.

Possible solutions:

• It could search back through its commits to find the latest commit that's also on another branch, and set that as the destination. When you create a pull request from the web client, it seems to do something like that.
• Simply default to dev or development, checking that they exist, before defaulting to master