add Google Test library
add TAP listener library for Google Test

see https://github.com/kinow/gtest-tap-listener
https://github.com/kinow/gtest-tap-listener/commit/b0c1a440f9299f0cbc686ce8d6d00b705e33348d
install TAP header for Google Test
start pretty-printing library for VHDL output
pprint: tests pass for Empty and Text
pprint: VCat tested
add Google Mock library
add gmock dependencies
split big test file
use gmock to test stream operators
use special methods for tests instead of friend declarations
fix bug in pprint::Text
first implementation of HCat; doesn't work, yet
pprint::HCat works
add pprint::PrettyPrintedWithChildren
add more tests
add PrettyPrintBuilder
try pretty-printer with vhdl::UsedLibrary
add seperateBy to pretty-printer
add fine-grained header dependencies to Makefile of vhdlgen