This lets us add the installed helper scripts to the cmake module
path, letting CMake users just include them after requiring Catch2
package.

This also goes for pkg-config installed by our CMake installation.

This includes

* Updating CMake version on Travis
* Adding a `Catch2` subfolder to the `single_include/` folder to
provide this include path both _inside_ the repository, and _outside_.
* Updated examples to build with the new paths
* Other general CMake cleanup

Android apparently does not support `std::to_string`, so we add a
small polyfill over it. Right now only the ULP matcher uses it,
but we have had plans to use it in `StringMaker<int>` and friends,
as it performs a lot better than `std::stringstream` based
stringification on MSVC.

See #1280 for more details

Unlike the relatively non-invasive old way of capturing stdout/stderr,
this new way is also able to capture output from C's stdlib functions
such as `printf`. This is done by redirecting stdout and stderr file
descriptors to a file, and then reading this file back.

This approach has two sizeable drawbacks:
1) Performance, obviously. Previously an installed capture made the
program run faster (as long as it was then discarded), because a call
to `std::cout` did not result in text output to the console. This new
capture method in fact forces disk IO. While it is likely that any
modern OS will keep this file in memory-cache and might never actually
issue the IO to the backing storage, it is still a possibility and
calls to the file system are not free.

2) Nonportability. While POSIX is usually assumed portable, and this
implementation relies only on a very common parts of it, it is no
longer standard C++ (or just plain C) and thus might not be available
on some obscure platforms. Different C libs might also implement the
relevant functions in a less-than-useful ways (e.g. MS's `tmpfile`
generates a temp file inside system folder, so it will not work
without elevated privileges and thus is useless).

These two drawbacks mean that, at least for now, the new capture is
opt-in. To opt-in, `CATCH_CONFIG_EXPERIMENTAL_REDIRECT` needs to be
defined in the implementation file.

Closes #1243

Catch2's documentation promises that listeners are called _before_
reporters, but because of the previous implementation, they were
called _after_ reporters. This commit fixes that.

Closes #1234