Change parsing of months. Use M for months and m for minutes

[nclaeys]

Changed the parsing in order to handle M as a pattern for months and m as a pattern for minutes. Added tests to validate that the parsing is done correctly.

• test to parse 1M
• test to parse 6M 1d and stringify it back
• test to stringify 6 * MONTH + HOUR and back

-> made the pattern case sensitive and verified that the tests still pass