[Pagination] Reactive calculation of sliceStart

[pixlo]

Fixes #86

Changes proposed in this pull request:

• Replace watch function with method.
• Calculating sliceStart on creating of instance.
• Watching all three props that actually affecting sliceStart variable.

@wxsms

[coveralls]

Coverage increased (+0.006%) to 96.765% when pulling 0b0cc77 on pixlo:pagination_reactive_recalculation into a036abd on wxsms:master.

[wxsms]

Thanks!