Streamline the build

[kkristof]

Currently the build instructions are lacking some information about which version of JerryScript and Gepard are needed. Also the experimental Gepard function define should be moved to the cmake instead of the gepardBinding.h

[kkristof]

In PR #4 the definition is moved to the cmake.

[kkristof]

For issue #2 I'm creating a script which makes possible to build the source from scratch, without using the experimental features of the Gepard. As the extra functions would be useful for experienced developers only, I think we can close this once the aforementioned issue has been closed.