Correctly validate the project prompt input.

It was possible to enter alpha characters, numbers out of range of the
project list, or nothing when prompted to select a project. The prompt input
will now correctly validate that input was added, and that it is a number within
range of the available projects.

Related to issue #9.
Merge pull request #10 from onmodulus/bug/project-prompt-validation

Correctly validate the project prompt input.