Scrub more variables when spawning java processes.

This adds scrubbing for `_JAVA_OPTIONS` and `JAVA_TOOL_OPTIONS`
which will quasi-silently add trumping jvm options to the command line
when present.  This leads to hermeticity leaks that can affect
correctness and are hard to debug.

Testing Done:
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/java:executor --test-pytest-options="-vs"
...
17:14:57 00:02   [test]
17:14:57 00:02     [pytest]
17:14:57 00:02       [run]
                     WARNING:root:Scrubbing CLASSPATH=/tmp/tmpnitp8W
                     WARNING:root:Scrubbing _JAVA_OPTIONS=-target 6
                     WARNING:root:Scrubbing JAVA_TOOL_OPTIONS=-Xmx1g
                     ============== test session starts ===============
                     platform linux3 -- Python 2.6.8 -- py-1.4.20 -- pytest-2.5.2 -- /home/jsirois/dev/3rdparty/pyenv/versions/2.6.8/bin/python
                     plugins: cov, timeout
                     collected 4 items

                     tests/python/pants_test/java/test_executor.py:47: SubprocessExecutorTest.test_not_scrubbed PASSED
                     tests/python/pants_test/java/test_executor.py:50: SubprocessExecutorTest.test_scrubbed_classpath PASSED
                     tests/python/pants_test/java/test_executor.py:54: SubprocessExecutorTest.test_scrubbed_java_options PASSED
                     tests/python/pants_test/java/test_executor.py:57: SubprocessExecutorTest.test_scrubbed_java_tool_options PASSED

                     ============ 4 passed in 0.15 seconds ============

17:15:02 00:07     [junit]
17:15:02 00:07     [specs]
               SUCCESS

Bugs closed: 260, 264

Reviewed at https://rbcommons.com/s/twitter/r/583/
Add AndroidTarget and AndroidBinary to pants

Several of the fields are passed default values, which will later be
either defined elsewhere or will be filled by the future
AndroidManifest parser (which will fill fields like target_sdk and package).

AndroidBinary has little difference from AndroidTarget, but was created
with future support of AndroidLibrary in mind.

Testing Done:
./pants goal test tests/python/pants_test:all and ci.sh both passed.

Adding pants.backend.android to base/dev_backend_loader causes tests to fail, so that has been left out of this patch for now.

Reviewed at https://rbcommons.com/s/twitter/r/572/
Remove unused import from backend.android.register.py

Testing Done:
ci.sh ran successfully.

Reviewed at https://rbcommons.com/s/twitter/r/589/
doc doc.py

a page about 'page' and 'goal markdown'

gave src/java/examples/hello a README.md and a corresponding BUILD target

tweaked a couple of doc pages that "included" hello/main/BUILD for a code example: tweaked them so that they wouldn't mention the page target; would be strange in that context.

Bugs closed: 257

Reviewed at https://rbcommons.com/s/twitter/r/582/
I don't have much idea (hehehe) what I'm doing, but it is working more than it was before.

Idea files are now actually generated and IntelliJ is opened. Codegen also appears to work now, after I moved some code out of ide_gen's __init__ and into its execute(), so that the code actuall gets generated before its called for. External jars work now. I've been testing it out on the jaxb examples and tests.

The code also currently has a mess of print statmements and commented out debugger invocations, as I've been trying to decipher how the old system worked and how the new system is supposed to work.