small performance-fix for minute-scaled slots

[ClemensSchneider]

two performance optimizations:
a) clone Date only once for minMinute and maxMinute checks
b) use faster jquery expression to determine elements to be examined.

saves performance especially when computing calendars with small slotMinutes values (e.g. 1)

[MattiSG]

This breaks on my usage (slotTable.find('tr').eq(slotI).find('td div')[0] yields undefined, but reverting the selector only is not enough).

Sorry, difficult to give any more details, the web app is closed source, and there are many other changes to examine in @ClemensSchneider's fork.

[MattiSG]

Oops, nope, sorry, this PR is valid. The fail was coming from timeposition_seconds_based.

[arshaw]

the date copying fix is a micro-optiization that won't help. the the selector fix actually makes things slower because it breaks a single selector-engine query (which will likely be handled natively) into multiple parts (which are handled by jquery instead).

[maxgalbu]

The selector fix is actually faster because :eq is not a native selector

• http://stackoverflow.com/questions/8401293/why-is-jquery-tdeq0-slower-than-td-eq0
• http://jsperf.com/jquery-eq-method-vs-eq-pseudo-selector

[arshaw]

ah, ok, you learn something new every day. I'll try to include this optimization in the next release

[arshaw]

i committed 6da36c7 to the upcoming release branch

[arshaw]

just released in v1.6.3