this commit adds a new page to the docs about the various
ways of defining quantity objects in Pint, and moves some
of the content from the tutorial in there

not certain if it's a better arrangement, the theory is
that a more concise tutorial with links to more detailed
pages might get users up and running faster

very open to suggestions though

giving the tutorial page on the docs a once-over for broken
Sphinx links

also:
- rewrote some sections for grammar
- changed (added) some section titles
- wrapped some long lines

some configurations in Travis skip optional dependencies,
which causes the doctests that rely on them to fail.

This fixes that by checking for those dependencies and skipping
the affected doctests with :skipif:

doctests updated for new 9-digit Quantity().__repr__()

added dependency on Serialize to requirements_docs.txt

in systems.rst: three errors remain, one may be a regression,
the others I'm not qualified to comment on

in nonmult.rst: some issues around floating point estimation are made
more robust by using print()

seems like Quantity().__repr__() changed since these
tests were written, and could benefit from some floating-point
truncation

in pitheorem.rst the only error was dictionary ordering, which
is now deterministic in python3.7, but may still cause
issues down the line

no doctests in getting.rst, marked all as code blocks

With this commit, all string formatting operations are locale aware

    >>> ureg.set_fmt_locale('fr_FR')
    >>> str(accel)
    '1.3 mètre par seconde²'
    >>> "%s" % accel
    '1.3 mètre par seconde²'
    >>> "{}".format(accel)
    '1.3 mètre par seconde²'

It should not break any code as the default locale value for the
Registry is `None` (meaning do not localize).

Close #984