I have never head of \l and MSVC warns about unknown escape sequence, so
I changed it into \f that definitely exists and potentially might be
useful.

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

This means that all tabs used in indentation are now 4 spaces and that
there should be no more trailing whitespace.

Ill also look into creating a pre-commit hook that will prevent this
from happening in the future.

Fixes #105

* 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.

(replaces need for PRs #310 and #504)

- and print it in *addition* to the decimal form

now matches behaviour of unsigned int