Set WARNINGS=1 instead of CHPL_DEVELOPER in nightly.

CHPL_DEVELOPER was added to nightly in commit 06088fc9. Based on internal
notes, it appears CHPL_DEVELOPER was added to enable additional C compiler
warnings. With pull request #594 those warnings can now be enabled by setting
WARNINGS=1 when calling make.
Update CHPL_DEVELOPER.txt docs.
Merge branch 'master' of github.com:chapel-lang/chapel into use-warnings-var-in-nightly

Conflicts:
	doc/developer/bestPractices/CHPL_DEVELOPER.txt
Remove CHPL_DEVELOPER from common.bash.
Add WARNINGS=1 when calling make in smokeTest.
Add second config to travis that builds with nightly test settings.

There are now two modes that travis will run with each commit:

1) Developer mode: Set CHPL_DEVELOPER=true and call `make check` with no
   additional arguments.

2) Nightly testing mode: Add DEBUG=0 WARNINGS=1 OPTIMIZE=1 when calling `make
   check` like the nightly testing currently does.
Improve smokeTest logging so devs better understand what environment was used.
Merge pull request #628 from thomasvandoren/use-warnings-var-in-nightly

Set WARNINGS=1 instead of CHPL_DEVELOPER in nightly.

CHPL_DEVELOPER was added to nightly in commit 06088fc. Based on internal
notes, it appears CHPL_DEVELOPER was added to enable additional C compiler
warnings. With pull request #594 those warnings can now be enabled by setting
WARNINGS=1 when calling make.

It is also worth mentioning that start_test explicitly unsets CHPL_DEVELOPER
and adds --cc-warnings flag to the compopts. So, start_test is already running with
backend C compiler warnings.

Also add an additional test configuration to travis. In addition to testing with
CHPL_DEVELOPER=true (developer mode), also test in "nightly test mode". "nightly
test mode" does not set CHPL_DEVELOPER, but it does set DEBUG=0
WARNINGS=1 OPTIMIZE=1 when calling make.