Confirmed that it improves perfomance of simple "for i in range(N): pass"
loop by 15% on Core2.

Remove some duplication in the code for generating
qstrdefs.generated.h and py-version.h

The compiler treats `if (MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE)` as
a normal statement and generates assembly for it in degug mode as if MICROPY_ERROR_REPORTING
is an actual symbol instead of a preprocessor definition.
As such linking fails because mp_arg_error_terse_mismatch is not defined when
MICROPY_ERROR_REPORTING_TERSE is detailed or normal.

- Use a single file env.props for defining the main directories used when building.
  env.props resolves the base directory and defines overridable output directories,
  and is used by all other build files.
- Fix the build currently failing, basically because the preprocessing command for generating
  qstrdefs uses different include directories than the build itself does.
  (specifically, qstrdefs.h uses #include "py/mpconfig.h" since the fixes for #1022
  in 51dfcb4b, so we need to use the base directory as include directory, not the py dir itself).
  So define a single variable containing the include directories instead and use it where needed.

This reduces dependency on assembler, and allows to consolidate global
variables in the future.

Addresses issue #1022.

Add a command for converting the WAV files to the amp skin tutorial, so that people can use their own files easily.

We are not word-for-word compatible with CPython exceptions, so we are
free to make them short but informative in order to reduce code size.
Also, try to make messages the same as existing ones where possible.

Reduces code size when MICROPY_ERROR_REPORTING_TERSE is selected.

- remove single string initialization style
- take list of strings instead
- store list in the type for fast lookup

Addresses issue #1027.

To make mp_obj_new_int_from_float() somehow work.

This fixes conversion when float type has more mantissa bits than small int,
and float value has small exponent. This is for example the case of 32-bit
platform using doubles, and converting value of time.time(). Conversion of
floats with larg exponnet is still not handled correctly.

We don't have an explicit ChangeLog file, but don't really need one
because we use a good version control system.  This script is useful if
you need a pretty-printed ChangeLog for some reason.