Some refactoring of existing project tree

[vitcpp]

• Source files were moved into the src subdirectory
• Implemented propagation of version number into pg_sphere_version function
• Some cleanup in Makefile

[vitcpp]

Rejected

[esabol]

Are you planning to submit a separate PR for the PGSPHERE_VERSION stuff that was in this PR (in order to address issue #10)? I hope so. 😄

[vitcpp]

@esabol, yes, I will do it in a separate PR. Thank you!