Cmake install

Edit: Need to re-think this, using h2o as a submodule in my project for now

I added cmake directives to install the library system-wide, I was also receiving the following warning when running cmake. I opted to rename test to h2o-test but you could keep it test and set the cmake policy to use the old policy.

```
Make Warning (dev) at build/cpm_packages/modules/github_h2o_h2o_origin_master/src/CMakeLists.txt:112 (ADD_CUSTOM_TARGET):
  Policy CMP0037 is not set: Target names should not be reserved and should
  match a validity pattern.  Run "cmake --help-policy CMP0037" for policy
  details.  Use the cmake_policy command to set the policy and suppress this
  warning.

  The target name "test" is reserved or not valid for certain CMake features,
  such as generator expressions, and may result in undefined behavior.
This warning is for project developers.  Use -Wno-dev to suppress it.
```