Support decoding Binary, Octal and Hex numbers as integers

[cowtowncoder]

(note: support for Hex numbers covered by #71 too, but this is the main catch-all issue)

Jackson 2.11 and before do not support alternative integer representations than standard decimal -- and actually incorrectly decode Octal ones. These representations should be supported.

NOTE: this will leave one rarely used type -- 60-based numbers -- unsupported. Reason for this, at least initially, is to avoid likely issues with backwards compatibility: these numbers are likely to conflict with notations used for ethernet (MAC) addresses, time values. Support may be considered at a later point.

[cowtowncoder]

Also: might as well support underscores while doing this.