Add failing test for ivy/maven cross-over configuration issue.
Fix Maven configuration mappings in Ivy.

It turns out there was a very subtle, and evil, issue sitting the Ivy/maven configuration, and it
related to dependency mapping.    A mapping of `foo->bar(*)` means that the local configuration
`foo` depends on the remote configuration `bar`, if it exists, or *ALL CONFIGURATIONS* if `bar`
does not exist.   Since the default Ivy configuration mapping was using the random `master`
configuration, which AFAICT is NEVER specified, just an assumed default, this would cause leaks
between maven + ivy projects.

i.e. if  a maven POM depends on a module denoted by an ivy.xml file, then you'd wind up accidentally
bleeding ALL the ivy module's configurations into the maven module's configurations.

This fix works around the issue, by assuming that if there is no `master` configuration, than the
maven default of `compile` is intended.   As sbt forces generated `ivy.xml` files to abide by
maven conventions, this works in all of our test cases.   The only scenario where it wouldn't work
is those who have custom ivy.xml files *and* have pom.xml files which rely on those custom ivy.xml files,
a very unlikely situation where the workaround is:  "define a master configuration".

Includes a test demonstrating the issue.
Fixes from review, better names and internal code documentation.
Add notes and add note about notes in Contributing.
Merge pull request #1586 from sbt/wip/cross-ivy-maven-issues

Fix ivy-mvn cross-wiring issues