Fixes mode assumption mismatch with h5py

The default opening mode is 'r+' which is actually read/write, this patch
changes the default behavior so that when files are opened with no mode
explicitly specified they are opened in read-only without the use of an
exclusion lock.

This patch may cause unforseen issues if readers depend on write privileges;
there is no evidence to suggest that they do.
Patches a testing utility to open files with correct permissions
Merge pull request #106 from lukecampbell/mode

Fixes mode assumption mismatch with h5py