Use options for default of parameter "shared".

[privefl]

I experienced problems with my shared memory which was full.
I think using shared = FALSE could be a mean to avoid this when there are repetitive definitions of big.matrix objects.
In order to be seamless for users, I created a new options bigmemory.default.shared with default value TRUE (as before for parameter shared) and use this option as the default value of parameter shared in R functions.
So, people who would want to use shared = FALSE as a standard could do it by changing the option, not all their calls to big.matrix().

[coveralls]

Coverage increased (+0.7%) to 53.543% when pulling 5cc6c66 on privefl:master into 0ffe325 on kaneplusplus:master.