Fix #24: default and property change

Default description was less than informative, now it;s
changed to project.GAV. Also, property name changed
from POM conflicting description to stagingDescription.
Merge remote-tracking branch 'origin/master' into issue-24
Added mapping to staging steps.

If set, the `stagingDescription` will override everything. This switch is
meant for CLI use (like `-DstagingDescription=...`).

Introduced stagingDescriptions (plural!) Map, that might be configured
in POM, and provides full mapping for staging actions.
Removed now redundant field.

That was introduced in some prev commit
Merge pull request #26 from sonatype/issue-24

Fix #24: default and property change