Uniform parser 'prereqs' file

This change-set applies a few white-space adjustments to `cmake/Modules/opm-parser-prereqs.cmake` in order to blend in with the conventions of other `*-prereqs.cmake` files under the OPM umbrella.

Furthermore, we also install a component search for Boost.Iostreams.  Although superfluous for the purpose of opm-parser, this enables using the same `opm-parser-prereqs.cmake` file in all modules.  The requirement for Boost.Iostreams comes from the (far removed) downstream module opm-benchmarks which uses Boost.Iostreams to handle (GZip) compressed input streams.

I appreciate that this is contentious.

Tested on (Debug and Release modes):
 * Ubuntu 12.04 LTS x86_64 (CMake 2.8.7, GCC 4.6.3, Boost 1.46.1)
 * CentOS 5.10 x86_64 (CMake 2.8.10.1, GCC 4.4.7, Boost 1.48)