Add specs for mock actors.
Implement MockSpecializedLiftActor and MockLiftActor.
Expose the mailbox of a Mock*LiftActor via the messages method.
Made run-mode auto-detection customisable.

This allows users to compliment or override the default logic used to
determine the run-mode (ie. dev/test/staging/etc) in absence of a
"run.mode" system variable.

An example use case would be a user using test framework that Lift
doesn't know about, and it undesirably running in development mode.
Use test run-mode when running tests from ScalaTest or IDEA.
Signed contributors.md
Split PropertyWithModificationCondition.set() into multiple lines and added doco.
Removed PropertyWithModificationCondition.
Added tests for run-mode property modification.
Moved Mailer's inner classes from trait to object.

Allows any class extending the Mailer trait to accept the same instances
of From(), To(), Subject(), etc.
Fixed MailerSpec after moving Mailer inner classes from trait to object.