This CL fixes the following issues:
* The Travis CI had the ctest invocation followed by a ";", so non-zero
  exit codes (indicating test failures) did not cause the build to fail.
* The AppVeyor CI had the ctest invocation followed by a ";", causing an
  error on Windows, where "&" plays the role of ";" [1].

The Windows CI (AppVeyor) will still be red after this CL, as some of
the tests are failing. However, this CL is a step forward, as it gets us
from failing to start tests to running tests and recording success/error
states.

[1] https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-xp/bb490954(v=technet.10)#using-multiple-commands-and-conditional-processing-symbols

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=236765633

This change adds a native Windows port (port_windows.h) and a
Windows Env (WindowsEnv).

Note1: "small" is defined when including <Windows.h> so some
parameters were renamed to avoid conflict.

Note2: leveldb::Env defines the method: "DeleteFile" which is
also a constant defined when including <Windows.h>. The solution
was to ensure this macro is defined in env.h which forces
the function, when compiled, to be either DeleteFileA or
DeleteFileW when building for MBCS or UNICODE respectively.

This resolves #519 on GitHub.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=236364778