* Correct/clarify build/install instructions

GTest is google test, don't obsfucate needlessly for newcomers.
Adding google test into installation guide helps newcomers.
Third option under this  line: "Note that Google Benchmark requires Google Test to build and run the tests. This
dependency can be provided three ways:"
Was not true (did not occur). If there is a further option that needs to be specified in order for that functionality to work it needs to be specified.

* Add prerequisite knowledge section

A lot of assumptions are made about the reader in the documentation. This is unfortunate.

* Removal of abbreviations for google test