clean up #ast_info.pre_render_asts

it is no longer used.
erlydtl_beam_compiler: refactor custom_forms for readability..
fix #157: avoid broken R16B03
R15B03-1 not available on travis..
drop bitdeli badge.
avoid R15B and R15B01

merl depends on changes in erl_syntax introduced in R15B02: https://github.com/erlang/otp/commit/53746070b5de7d1e9f809d516648948b0a246561
Run merl tests together with erlydtl tests.

Avoid always running the merl tests when only compiling.

cf. richcarl/merl#6
make -> gmake typo for freebsd
fix #153: parse (and unparse) comment tags {# .. #}

call the {# .. #}-construct for comment tag (previously comment_inline).
n.b. {% comment %} ... {% endcomment %} is a comment block.

some cleanups in sources_parser + handle the new comment_tag token.

beginnings of tests for the unparser (currently lives in the sources parser test suite).
support translation context in `trans` tags (#131)
support translation context in `blocktrans` blocks (#131)
support count and plural forms in blocktrans blocks (#131)

open question: we ought to support multiple plural tags
  to support languages that have more than two variations
  dependent on the count.
fix blocktrans bugs (#131)
test `_(Value)` translations (#131)

Support trans_fun/1 as compile time translation fun too..
set current locale to 'default' if not specified (#131)
align compile vs render options for translation (#131)

Print warning message to stderr on unclosed variable in block translation.
Preserve _ prefixed to variable names when generating erlang

Completely backwards compatible, but means we can enable
warnings_as_errors, and name unused variables with an underscore.
(see 2 new tests)
Merge branch 'unused-vars-fix' into 0.9.3-irccloud