This reverts commit a189387f49f129a3554c47426a0dfe176c5c0781.

Character encodings that are not valid in XML 1.0 are instead written using C-style escapes

In reality, this is a relatively small performance improvement,
especially with the previous improvements removing lots of superfluous
string handling, but still was measurable.

* Empty strings are now direct constructed as `std::string()`, not as empty string literals.
* `startsWith` and `endsWith` no longer construct new a string. This should be an improvement
for libstdc++ when using older standards, as it doesn't use SSO but COW and thus even short
strings are expensive to first create.
* Various places now use char literal instead of string literals containing single char.
** `startsWith` and `endsWith` now also have overload that takes single char.

Generally the performance improvements under VS2015 are small, as going from short string
to char is mostly meaningless because of SSO (Catch doesn't push string handling that hard)
and previous commit removed most string handling if tests pass, which is the expect case.

This commit fixes the following scenario:
  * You have a test that compares strings with embedded control
  characters.
  * The test fails.
  * You are using JUnit tests within TeamCity.

Before this commit, the JUnit report watcher fails on parsing the XML
for two reasons: the control characters are missing a semicolon at the
end, and the XML document doesn't specify that it is XML 1.1.

XML 1.0 --- what we get if we don't specify an XML version --- doesn't support embedding control characters --- see
http://stackoverflow.com/questions/404107/why-are-control-characters-illegal-in-xml
for all of the gory details.

This is based on PR #588 by @mrpi

(replaces need for PRs #310 and #504)

- don't encode apostrophes
- only encode quotes in attributes
- encode control characters (as in PR #465)