Do not fail when `createSrc` set is too narrow.

It seems like the original code is rather suspicious (as it
fails with empty message) so I presume that the intended behavior
was to *not* fail when `createSrc` set is too narrow.

Fixes #139.
Made `createSrc` and `eclipseOutput` keys scoped properly.

When resolving `createSrc` and `eclipseOutput` keys we take into
account configuration so both of those keys can be scoped properly.

This allows one to have e.g.:

EclipseKeys.createSrc in Test := EclipseCreateSrc.ValueSet.empty

which makes sbteclipse to not include test directories in generated
.classpath file.

Updated a scripted test to show the effect of this change.
Merge pull request #141 from gkossakowski/create-src

Fixes related to `createSrc` key