Use password instead of passphrase

Previously, the aws-maven wagon used the username and passphrase
facilities in the wagon API to contribute credentials.  At some point,
the implementation of the API changed such that it was no longer
possible to use that pairing together.  This change updates the wagon
to use username and password now, which can be used together.

[#14]
Merge branch 'jmena-master'
Java 7 minimum

Since Java 6 does not have the proper CAs to handle S3's SSL
certificates, there's no longer any need to support Java 6.  This
change updates the minimum supported Java to Java 7.