Convert README to Markdown.

Conflicts:
	README.md
Add Travis CI build status badge to README.
Travis CI integration.
Test against 5.2.3, our minimum version.
Also test against 5.3, 5.4 and 5.5 to be forward looking
Travis doesn't have PHP 5.2.3.  Test against 5.3 / 5.4 / 5.5.
Follow-on for #2043.  Fix some tests that made bad assumptions about
the state of the world.  These were uncovered when we ran the tests on
Travis which runs the tests in a different order than what I (and
apparently others) use on their dev boxes.
#2040 - Deactivate modules that no longer exist.
- added module::deactivate_missing_modules()
- revised module::deactivate() to change log message if the module is missing
- added call to new function in module::get_obsolete_modules_message()
- added call to new function when loading admin/maintenance menu
Automated update of .build_number to 273 for branch 3.0.x
Last update: c4cc6dc50bf14207a0876492947b568b8c87559c (17 commits ago)
Set exit status to the number of failed tests.  This should fix issues with Travis.
Automated update of .build_number to 274 for branch 3.0.x
Last update: 52c142e5bd141d6668a49b11dba8bed8d0cb48b9 (1 commits ago)
[ci skip]
Follow-on fix: count errors as well as test failures.
Automated update of .build_number to 275 for branch 3.0.x
Last update: 4d87c0c7496e98d69ddf997d4b54023189da149f (1 commits ago)
[ci skip]
Only build 3.0.x, 3.1.x and master branches
Automated update of .build_number to 276 for branch 3.0.x
Last update: eb06f176864e031928e15501b78953abafc3df09 (1 commits ago)
[ci skip]