Harmonized strptime() with GNU R

GNU R's implementation of strptime() ignores trailing
characters after the format string.

That is,
strptime("2012-12-31 adfasdfsdf", "%Y-%m-%d")

evaluates to "2012-12-31"
[zoo package] Fixed handling of zero-length symbols
[zoo package] Removed graphics-related stubs from maven test runner

This was causing problems because it set the environment of plot() to
the empty environment rather than the graphics namespace. we have
since stubbed out plot() in the graphics package anyway