FscHelper: separate `fsc` function with exit code, &c.

This commit contains several small-ish changes. In order of appearance:

  - Use an `FscParams.Default` static member instead of a free variable,
    to express the tight coupling of the defaults with the type itself.

  - Print compile errors and warnings with the appropriate `trace...`
    functions.

  - Return an exit code from the `fsc` function as I intend to use it to
    help implement incremental build logic. The `Fsc` function
    (uppercase 'F') still doesn't return anything so it can continue to
    be used in the existing framework.

The `Fsc` call syntax looks very similar to what was originally proposed
in #235.
Merge pull request #491 from yawaramin/issues/235

FscHelper: separate `fsc` function with exit code, &c.
Run cleanup over FSCHelper
Make the scs local - do not reuse in multiple compile runs
Some more cleanup in the FSCHelper
Adding FSC helper to release notes
Bump version to 3.1.0
Add description for FSCHelper
Merge branch 'master' of https://github.com/fsharp/FAKE
Try to standardize Fsc docs
Try to standardize docs
GuardedAwaitObservable was made public by accident - this was fixed