Add openmp_version method

[kiwiroy]

This pull request adds the openmp_version method and a useful value for the Alien::Base::version method. Both provide access to the version of OpenMP available to the system to satisfy #21.

The format of the version is different is each case.

1. openmp_version - the value of _OPENMP i.e. 200505
2. version - a decimal equivalent translated using this table.

[coveralls]

Pull Request Test Coverage Report for Build 2318853541

• 10 of 10 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.8%) to 86.441%

---

Totals
Change from base Build 1505030466:	2.8%
Covered Lines:	51
Relevant Lines:	59

---

💛 - Coveralls