Finish lager API modifications
Update config API to handle multiple sinks
Identify areas where new code is yet to be written

First pass at adding sink parameter into the external API layer.

TODO:
* config updates
* loglevel changes per sink

in common test, starting and stopping applications can make your logs
very noisy.  the patch allows these messages to be suppressed on the
happy path, and raises the high water mark during ct tests so that
messages aren't dropped.

Note that compiling on OTP 17.0, or later, will produce beam files
(specifically lager_trunc_io) that are incompatible with R16B03-1, or
earlier, due to erlang:is_map/1 using a new beam opcode (156). However
if compiled on R16B03-1, or earlier, will produce beams files that are
compatible with 17.0, and later, and maps will be formatted where
supported by the active OTP release.

* Fix unmatched return
* Fix set_loglevel call to store mask in state
* Add common_test to plt for ct:pal spec

the default handlers defined at lager.app overwrites handlers defined previously by application:set_env(lager, handlers, ...) if lager application is started application:start(lager)

Change a latin1 sequence so it is given as a list of integers. this avoids the problem entirely
since ASCII has the same representation in both latin1 and in utf-8

Previously all messages were being flushed, which meant that trapped exits
and internal gen_event messaging was also flushed, leading to undefined
behaviour.

closes #198

Modified unit test to test 0 size tuple case.

Proplist module is a lot slower than lists:keyfind, which is a BIF,
because proplists has to work with 'bare' atoms as well as 2-tuples.

This should marginally improve the throughput when printing many
error_logger messages.

The fix for this was merged in R16 somewhere, so this workaround is no
longer needed, and fixes the EQC test from spotting the divergent
behaviour with io_lib.

See also: http://erlang.org/pipermail/erlang-patches/2012-September/003034.html