new param for page target, links. List of hyperlinked-to targets

List of hyperlinked-to targets. We use dependencies for this now.
But circular dependencies are an error while hyperlink-cycles should be OK.

So param page(links=[...]) feeds into traversable_specs (versus traversable_dependency_specs).

While I was looking at the tests, un-commented-in some commented-out assertions that had a done TODO.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/39520576

Bugs closed: 253, 742

Reviewed at https://rbcommons.com/s/twitter/r/1242/
Migrate some utility tasks to new-style options.

Reviewed at https://rbcommons.com/s/twitter/r/1248/
fix [run.junit] -> [test.junit]

Testing Done:
https://travis-ci.org/jinfeng/jinfeng-pants-fork/builds/39534255

Reviewed at https://rbcommons.com/s/twitter/r/1256/
fix the error msg in favor of the new style arg name

Reviewed at https://rbcommons.com/s/twitter/r/1258/
WikiArtifact instances also have provides; limit ivy to jvm

This should never get hit unless you accidentally do goal compile on a
wiki -- but maybe you do compile ...:: and hit it.

Testing Done:
Ran tests.

Reviewed at https://rbcommons.com/s/twitter/r/1259/
Refactor artifact cache exceptions.

Presently, the artifact cache will treat any exception during the
insertion process as a non-fatal error.  But some errors should be
fatal -- any sort of misconfiguration, or an attempt to cache
nonexistent files, or an exception thrown while moving files around.
Also remove ArtifactCache.CacheError, as it was not clear when
to use ArtifactCache.CacheError and when to use ArtifactCacheError.

Reviewed at https://rbcommons.com/s/twitter/r/1261/
included code snippets: don't just slap in a pre. syntax highlighting

The pygments code-syntax-highlighting code has some built-in functionality to guess at a code snippet's language based on filename and content. We hadn't hooked it up. This hooks it up.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/39542386

patched this and https://rbcommons.com/s/twitter/r/1191/ into a branch and test-generated the doc site: http://pantsbuild.github.io/sitegen-test/JVMProjects.html

Bugs closed: 693

Reviewed at https://rbcommons.com/s/twitter/r/1182/
Fix error message: add missing space

Reviewed at https://rbcommons.com/s/twitter/r/1266/
Singleton for requests session

Instead of instance-level, use a module level singleton so that in multiproc code,
subprocesses each have their own, reusable session.