Add support for generating SHARED library with -DBUILD_SHARED_LIBS=ON flag

[PARTHSQL]

This change adds support for generating SHARED library which can be dynamically linked against any application.
When supplied with -DBUILD_SHARED_LIBS=ON the cmake command will generate a dynamically linkable library.
This change also updates README with this.

[tqchen]

Thanks for the contribution, may i ask the usecase of this? Ideally we would like to keep this project minimal so we only provide static link option

[PARTHSQL]

Thanks @tqchen looking at the PR. My application will have to support and update with any changes in this library if I statically link it. Making it dynamic library gives option to have modularity in my application, independent updates of this library, and also would be more memory efficient as this shared library will be used by multiple applications.

[PARTHSQL]

@tqchen can you help me with this PR?