Use DOMPDF_FONT_CACHE for new/remote fonts

So DOMPDF_FONT_DIR only contains the provided fonts and does not have to be writeable
Merge remote-tracking branch 'upstream/master'
Clarify composer instructions
Merge pull request #724 from adrianmacneil/patch-2

Clarify composer instructions
Merge branch 'master' of https://github.com/VIISON/dompdf into VIISON-master
Improve font management

DOMPDF_FONT_DIR can now be customized so that fonts can be stored
outside the dompdf installation directory. The fonts included
with dompdf are now hard-coded so that they are always available.
Updating distributed font family cache after font management update
Merge branch 'font-management-tweak'
Improve counter handling (fixes #440, #710)

If an element that increments a counter was rendered but pushed
to the next page because of lack of space the counter would be
incremented a second time. Now when splitting an element dompdf
will decrement counters that were incremented.

Also, nested counters would break depending on where the paging
took place. Because dompdf reset counters as it climbed up the
element tree higher levels of nested counters were being reset.
Now dompdf saves the current value of any counters on an element
that is split. When that element is re-rendered on the next page
the counter value is set according to the saved value.

Additional modifications improve improve handling of the use case
outlined in #705.
Fix counter type detection
Fixed issue when fs is numeric 0 due to PHPs loose typing in a switch statement. Undefined offset: 0 in include/style.cls.php on line 879.
Merge pull request #744 from 12hys/master

Force variable type to avoid false match in switch statement due to type conversion
Fix division-by-zero logic error
Don't decrement counters if the increment is on the body element
Account for possible color transparency
Update php-font-lib to 560856f8e83ea8def6214add2712711ac416591d
Improve MBString backfill (addresses #755)

Instead of checking for the existence of a particular function we
now check for the presence of the MBString extension. When defining
the MBString constants use the def() function. Rather than checking
for the presense of a single MBString function check every function
prior to creation.

The above changes were made in response to an incomplete MBString
backfill performed by WordPress.
Update php-font-lib class file reference (closes #751))

Patch courtesy of steelfox.
Capture generated content for font subsetting (#750)

Significant code updates will be necessary to capture the
value of generated content for use in the font subset
character list. For the time being we will use the content
definition from the stylesheet.
Update mb_detect_encoding function check