Add an option to ignore UTF-8 encoding errors

By default Jiffy is quite strict in what it encodes. By default it will
not allow invalid UTF-8 to be produced. This can cause issues when
attempting to encode JSON that was decoded by other libraries as UTF-8
semantics are not uniformly enforced.

This patch adds an option 'force_utf8' to the encoder. If encoding hits
an error for an invalid string it will forcefully mutate the object to
contain only valid UTF-8 and return the resulting encoded JSON.

For the most part this means it will strip any garbage data from
binaries replacing it replacement codepoint U+FFFD. Although, it will
also try and the common error of encoding surrogate pairs as three-byte
sequences and reencode them into UTF-8 properly.

Update rebar and the Makefile

  * Update rebar to a more recent version
  * Update rebar.config for deprecation warnings
  * Allow people to export the REBAR environment variable to use
    their own version of rebar
  * Fixed `make clean`

Fix segfaul when encoding with the uescape option

There was a bug that was resetting the extra amount of space needed when
encoding unicode characters with the `uescape` option enabled.

Fix uescapes for combining characters

I had a silly direction mistake in a bit shift that was causing the high
portion of all combining characters to be printed as \uD800 which is
obviously wrong. This bug only affects people using the non-default
uescape option during encoding.

Ensure that encoded doubles have a decimal point

Numbers like 1.0 were being encoded as <<"1">> which can lead to a bit
of confusion. This merely checks if a decimial point exists and if not
it appends ".0" to the value.

Allow iolists to be passed directly to decode/1

Fixes davisp#15

Fix uescape encoding bug

The encoding length for unicode values was applied to the wrong index
variable.

Thanks to Paul Guyot for the detailed bug report and fix.

Fixes davisp#7

Use git version info.