Disallow normalization and improve robustness against invalid input

[srishanbhattarai]

Changes

• Reject B.S. dates (via the Date() function) that would have normalized. (fixes #79)
• Add upper bound checks for date limits, don't know why this wasn't there already
• Go' iota based enums are very loosely typed so a nepcal.Month value of 64 was possible since they are just ints. Added a check for this.

Notes

There is more to #79 than may initially seem. (Read #81)

[coveralls]

Pull Request Test Coverage Report for Build 304

• 23 of 31 (74.19%) changed or added relevant lines in 3 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.7%) to 78.409%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
nepcal/util.go	12	14	85.71%
cmd/nepcal/cli.go	0	6	0.0%
<!--	Total:	23	31

Files with Coverage Reduction	New Missed Lines	%
nepcal/nepcal.go	2	75.27%
<!--	Total:	2

Totals
Change from base Build 302:	-0.7%
Covered Lines:	345
Relevant Lines:	440

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💛 - Coveralls