Refactored issue #29; addressed issue #1 with R_HOME

1) Addressed the issue #1 with R_HOME environment variable; if set it overrides R install detection over Registry discovery
2) Refactored all R installation discovery activities into a separate function selectRLocation and moved it into Helpers module of RInterop, where it should belong
3) Added a run-time check that R engine actually exists at location derived either from Environment, or Registry

Issues #1 and #29 in main repo can be closed upon successful review.