Mark sudo: required for Travis

Google Benchmark's Travis build currently requires "sudo" to install newer
versions of CMake and similar. See this for more details:

https://docs.travis-ci.com/user/workers/container-based-infrastructure/

Since Google Benchmark was put into Travis before 2015-01-01, it gets the
standard infrastructure implicitly, so sudo works. But anyone who forks this
repository and tries to add Travis.CI (so they can see if the build works
before creating a PR) gets broken builds before this change.

Fix MSVC++ command line warnings and support RELWITHDEBINFO / MINSIZEREL builds

Fix appveyor's older MSVC++ builds by working around 2015 Update 2 bugfix

MSVC++ before 2015 Update 2 has a bug in sleep_for where it tries to
implicitly += the input with a nanoseconds variable. Work around this by
using nanoseconds directly (which can be implicitly +='d with
chrono::nanoseconds).

In addition to release, CMake supports RELWITHDEBINFO and MINSIZEREL
build configurations. In particular, debug info is necessary for many
profilers to do anything useful, making RELWITHDEBINFO important here.

MINSIZEREL was added for completeness' sake.

BENCHMARK_ENABLE_LTO=true was completely replacing
CMAKE_CXX_FLAGS_RELEASE; meaning neither CMake's release defaults nor
user customizations were being applied.

This change looks for CMake's default setting for MSVC++, /W3 (and any
other level should that change in the future), and removes it before
adding /W4. This stops the build for MSVC++ emitting warnings about
/W4 overriding /W3 earlier on the command line.

Support for manual timing (see issue #198, https://github.com/google/benchmark/issues/198)

Only output optional fields if they're set

Close a memory leak in configuration code

Add section on iterations.

Also add some subheadings, and fix up the line lengths.

Fixes #194

Fix SetLabel usage example

Some benchmarks may run a few milliseconds which makes it kind of hard to visually compare, since the currently only available nanoseconds numbers can get very large in this case. Therefore this commit adds an optional command line flag --benchmark_time_unit which lets the user choose between ns and ms time units for displaying the mean execution time.

Fix missing include in map_test.cc

building with libc++.

Pass const State to Fixture::TearDown. Fix memory leak in fixture_test

Format json in README.md

Pass const state to Fixture::SetUp. Add map_test.

Add number of threads to State.

Having access to the thread count from within a benchmark is useful,
for when one wants to distribute a workload dynamically among the
benchmarks running in parallel e.g when using ThreadRange() or
ThreadPerCpu().