Accept a directory with -f/--file

The overwhelming majority of Maven POMs are in fact named pom.xml. Yet if you want to build a project in another directory, you are forced to specify e.g. -f subdir/pom.xml where /pom.xml ought to be obvious.

This simple patch lets you pass a directory to -f in which case the filename pom.xml is assumed. There should be no compatibility issue since 3.0.4 just rejects such paths with a silly error:

$ mvn -f /src/maven3 install
[INFO] Scanning for projects...
[ERROR] The build could not read 1 project -> [Help 1]
[ERROR]   
[ERROR]   The project  (/src/maven3) has 1 error
[ERROR]     Non-readable POM /src/maven3: /src/maven3 (Is a directory)

(I made an analogous change for Ant 1.8.3 in revision 1204655.)