* Add binary conformance tests for map fields

* Update failure list

* Fix php conformance test

* Fix php conformance test

In 32-bit platform, int64 should be string. However, map iterator returns string key as integer.

* Add more test cases for map

* Update failure list

* Test singular fields are encoded in canonical way

* Defautl values in proto3 should not be encoded.
* Values should be converted to the canonical representation (e.g.,
long int64 value may be truncated for int32 field)

* Update failure list

* Update failure list

* Fix empty FieldMask json encoding/decoding

* Add failed test to python's conformance failure list

* Give a unique category to each test.

This change introduce a TestCategory enum to ConformanceRequest. Existing tests
are divided into three categories: binary format test, json format test and json
format (ignore unknown when parsing) test. For the previous two categories, there
is no change to existing testee programs. For tests with the last category, testee programs
should either enable ignoring unknown field during json parsing or skip the test.

* Fix python test

* Fix java

* Fix csharp

* Update document

* Update csharp generated code

* Enable ignoring unknown in json parsing in php

* Update generated descriptor files

* Update failure list for other languages.

* Remove unnecessary php files

Add note about JSON tests maybe being wrong.

- Add note about the JSON test maybe not being correct yet.
- Add test to checks the generated names for double underscores to be sure they
  are what is expected.

* Add conformance test for zero fields in oneofs.
* Add failures to the "expected" files.

Python 2.x doesn't detect unpaired surrogates so we have to
do that manually.

We should now have no conformance failures.

The tests are run from Travis in the same way as on other
platforms. Currently some expected failures - but only expected
in that they're what we got to start with. Will try to fix them in
other pull requests.