Uniform API for load_XXX methods. Also add support for reading with stride and atom_indices to the remaining XXXTrajectoryFile class's read() methods

Close rmcgibbo/mdtraj#106

Squashed commit of the following:

commit d997090c95f4ea8949a7115fe59b4e01898335d9
Author: Robert McGibbon <rmcgibbo@gmail.com>
Date:   Sat Jun 29 22:37:47 2013 -0700

    added missing file

commit 87fe9fc30b9a68eccc461b3c6170f3637afebdf3
Author: Robert McGibbon <rmcgibbo@gmail.com>
Date:   Sat Jun 29 22:33:06 2013 -0700

    added two new test files to the suite

commit 16fa01198658bd09604aaaa5e193c792437965dd
Author: Robert McGibbon <rmcgibbo@gmail.com>
Date:   Sat Jun 29 22:19:51 2013 -0700

    All loaders now support the same stride and atom_indices kwargs, and load_xxx all take the same kwargs.