build: add install target to the makefile

Before this commit, there was no way to install libuv other than
manually copying required files to the path. This commit introduces
an install target which installs the libuv.a, libuv.so and header files
to the install prefix.

    ``
    make
    make install
    ``

The above will use the default prefix `/usr/local`

    ``
    make
    make install PREFIX=/usr
    ``

The above will install to `/usr`