Abandon usage of scala.App for 'main' method...

Motivation for this is to remove the 'sequential' modifier for tests...
concurrent testing against scala.App is not safe:

https://twitter.com/rtyley/status/340376844916387840

...I'd rather have faster tests than the limited convenience of
scala.App. Unfortunately, the BFG uses JGit's WindowCache config to
handle large commit objects - and that is *static* config, so
concurrent tests don't work well with it:

[info] Massive commit messages should
[info] ! be handled without crash (ie LargeObjectException) if the user specifies that the repo contains massive non-file objects
[error]     ExceedsLimit: Object d8875d55c4f723e4a7caf54fe8293abfd316f2e6 exceeds 5,242,880 limit, actual size is 10,486,015 (ObjectLoader.java:191)
[error] org.eclipse.jgit.lib.ObjectLoader.getCachedBytes(ObjectLoader.java:191)
[error] org.eclipse.jgit.revwalk.RevWalk.getCachedBytes(RevWalk.java:859)
[error] org.eclipse.jgit.revwalk.RevWalk.parseNew(RevWalk.java:823)
[error] org.eclipse.jgit.revwalk.RevWalk.parseAny(RevWalk.java:809)
[error] com.madgag.git.package$RichObjectId.asRevObject(package.scala:178)
[error] com.madgag.git.bfg.cleaner.RepoRewriter$$anonfun$2.apply(RepoRewriter.scala:83)
[error] com.madgag.git.bfg.cleaner.RepoRewriter$$anonfun$2.apply(RepoRewriter.scala:83)
[error] com.madgag.git.bfg.cleaner.RepoRewriter$.createRevWalk$1(RepoRewriter.scala:83)
[error] com.madgag.git.bfg.cleaner.RepoRewriter$.rewrite(RepoRewriter.scala:89)
[error] com.madgag.git.bfg.cli.Main$$anonfun$main$1.apply(Main.scala:60)
[error] com.madgag.git.bfg.cli.Main$$anonfun$main$1.apply(Main.scala:35)
[error] com.madgag.git.bfg.cli.Main$.main(Main.scala:34)
[error] com.madgag.git.bfg.test.unpackedRepo.run(unpackedRepo.scala:43)
[error] com.madgag.git.bfg.cli.MainSpec$$anonfun$13$$anonfun$apply$9$$anon$7$$anonfun$6.apply$mcV$sp(MainSpec.scala:83)
[error] com.madgag.git.bfg.cli.MainSpec$$anonfun$13$$anonfun$apply$9$$anon$7$$anonfun$6.apply(MainSpec.scala:83)
[error] com.madgag.git.bfg.cli.MainSpec$$anonfun$13$$anonfun$apply$9$$anon$7$$anonfun$6.apply(MainSpec.scala:83)
[error] com.madgag.git.bfg.test.unpackedRepo.ensureRemovalOf(unpackedRepo.scala:73)
[error] com.madgag.git.bfg.cli.MainSpec$$anonfun$13$$anonfun$apply$9$$anon$7.<init>(MainSpec.scala:82)
[error] com.madgag.git.bfg.cli.MainSpec$$anonfun$13$$anonfun$apply$9.apply(MainSpec.scala:80)
[error] com.madgag.git.bfg.cli.MainSpec$$anonfun$13$$anonfun$apply$9.apply(MainSpec.scala:80)