Clean up DataJoint project, fix #338 and #340, and reorganize package content

[eywalker]

Fixed #338 and #340

• If errors are encountered during matplotlib or pygraphviz import, dj.ERD is replaced with a stub function that issues warnings when invoked, fixing Make graphviz optional #338
• MANIFEST.in has been added to include requirements.txt into the source distribution, thereby fixing Fix source file distribution #340
• All Dockerfiles except for the main Dockerfile has been removed in accordance to the proposal in Migrate all DataJoint related Dockerfiles into a single repo #339
• Reflecting Make graphviz optional #338, pygraphviz and matplotlib are no longer part of strict requirements as spelled out in requirements.txt.
• A separate file test_requirements.txt listing packages that should be installed to fully test DataJoint (i.e. pygraphviz and matplotlib). .travis.yml has been updated to reflect this change.

[coveralls]

Coverage decreased (-0.2%) to 92.655% when pulling 3fba2dd on eywalker:master into cba4696 on datajoint:master.

[coveralls]

Coverage decreased (-0.2%) to 92.655% when pulling 984c3b7 on eywalker:master into cba4696 on datajoint:master.

[coveralls]

Coverage decreased (-0.2%) to 92.608% when pulling 1b7cc3c on eywalker:master into cba4696 on datajoint:master.