Added 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`

Shared libraries will be installed only if it is available.