fix various typos
conditional compile support for snappy

This adds CMake support for conditionally compiling snappy support for blosc.
Should still be able to copy and paste sources, since HAVE_SNAPPY is only
defined via config.h, but will need an empty config.h... Not sure how to solve
that one yet.
only run snappy benchmark if lib was compiled with snappy support
Merge pull request #34 from esc/conditional_snappy

Conditional snappy