package restructuring for nepcal package

[srishanbhattarai]

• Model it after the time package.
• There will be a central Time struct off of which we attach methods for all utilities. This should be fine because everything will be namespaced by the package name nepcal.Time
• This means that the public API needs to change
  • For e.g. these functions now become part of the Time struct.
  • Consult the methods that hang off time.Time for reference.
  • There can be a time.Time struct embedded inside nepcal.Time
• Any functions that aren't methods of the Time struct will (probably only) be constructors to create the structure.
  • For e.g the time.Date function will similarly be implemented as nepcal.Date with the same API. The caller does the extra business of argparsing, which is already the case.
  • Similarly, the nepcal.Unix function can be a similar equivalent.
• This imposes certain limitations as we cannot have functions to ask the package "What is the BS equivalent weekday of Wednesday?". This is probably silly to ask anyway. The struct can still expose the weekday.
• Change functions like totalDaysSpannedUntilDate to be implemented using methods

[srishanbhattarai]

Closing in favor of #48