Do not wait infinitely for checkers in tests

Signal an error if a checker did not complete after 15 seconds.  No
checker should take this long, so this definitely indicates an error in
the checker test.
Handle errors during error parsing

Provide better error indication upon errors, in a first reaction to #78.
Fix taglines
Use carton for dependency management
Merge pull request #79 from lunaryorn/carton

Use carton for dependency management
Do not attempt to parse an empty XML tree [#78]
Specify carton location for dependency installation
Test PHP CodeSniffer without errors [#78]
Document the Carton requirement of the test suite

[ci skip]