Use project-specific compiler settings when evaluating a worksheet.

No test since there is no easy way to trigger some observable effect from
compiler settings, other than using compiler plugins (and continuations are
missing from the 2.11 build).

* -deprecation or -feature won't work because warnings are not reported (see #174)
* the continuations plugin isn't part of 2.11, so tests would only work in 2.10

However, I think the integration test isn't really necessary: `scalacArguments`
is part of `sdt.core` and used by the sbt builder.