Add a favicon to the Buck docs.

Summary: The docs needed a favicon. Now it has one. :)

Test Plan: Loaded the docs, noted that the favicon is being rendered.
Set no_dx to take in a list of build rules as opposed to classpaths.

Test Plan: Unit Tests dawg
Remove '6' input from DefaultJavaLibraryRule.

Summary:
Fix DefaultLibraryRule to not consider sourceLevel and targetLevel as
inputs.  Unless a file named '6' happened to exist, this caused buck to
treat many build results as non-idempotent, resulting in significant
incremental build performance degradation.

Test Plan:
Verify that the following reports a build time under one minute:
  buck clean && buck build fb4a && time buck build fb4a
Fix SplitZip when no secondary dexs are created.

Summary:
When D773417 moved us to Guava, it changed ZipWithMaxDeflate to close instead of closeQuietly.  Native M Site and Appcenter both request to be splitzipped, even though they don't really need it for all builds.
This resulted in SplitZip producing no secondary dexes, which we then tried to zip with ZipWithMaxDeflate, which crashed when we tried to produce an emtpy zip.

Sorry for the reviewspam, we need to fix this.

Test Plan:
Unit test, built nativemsite locally.

testinprod
Update no_dx docs

Test Plan: My Eyes
Miscellaneous documentation fixes.

Summary:
@boval reported that the Quick Start instructions left IntelliJ
in a state where it associated a package named "java.com.example.activity"
with the android_library code rather than "com.example.activity."
This was because the Quick Start instructions failed to reference
the necessary update to .buckconfig.

Also fixed some spacing issues.

Test Plan: Inspect locally at http://localhost:9811/.
Fix an annoying typo in the overview docs.

Test Plan: visual inspection
Print out a custom error if using JRE without JDK.

Summary: I hate Java 7 so much.

Test Plan:
I think I used one of those scripts to install Java 7 on Ubuntu and I
ended up with the JRE but not the JDK and now life is bad.
Make `buck clean` synchronous.

Summary:
Had to refactor Process execution logic to safely take advantage of
Process.waitFor().

Test Plan: Sandcastle builds.
Change a RuntimeException to a HumanReadableException.

Test Plan: Violate visibility and verify how error is printed.
Remove unused warnings.

Test Plan: No yellow icon in Eclipse.