Remove include/lib requirements in Rakefile

If SFML is properly installed on Linux, there is no need to copy its
include and lib folders.
Make Linux build instructions simpler/agnostic

Reorder instructions to put all prerequisites (Ruby, SFML) first.
Mention `rake test` before `rake install`.

Make Ruby/SFML build instructions more agnostic. It is the user's
responsibility to figure out the best way to configure/install these
dependecies for their system. Arbitrary restrictions (e.g. "You must
compile Ruby from source") might not be true for certain systems.
Fix links for SFML source
Readd -fPIC (needed for building on (some) Linux)
Test latest build rather than installed build

Made require paths in tests relative to rbSFML directory. This tests the
latest build rather than the latest installed build. This is good since
you can test new builds without breaking an existing one.
Merge pull request #43 from silverhammermba/buildfix

Fix/Improve Build on Linux