Add U.first(iterable, pred, n) method and fix other U.first() methods.

[terhesb]

In this change I implemented a new first method, which return a list with the first n elements that satisty the predicate.
I also fixed a bug in the U.java file. At the 1138. row the Math.min(n, list.size()) call was missing.

[codecov]

Codecov Report

Merging #203 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master   #203   +/-   ##
=======================================
  Coverage       100%   100%           
- Complexity     1117   1119    +2     
=======================================
  Files             8      8           
  Lines          3414   3419    +5     
  Branches        657    657           
=======================================
+ Hits           3414   3419    +5

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/com/github/underscore/U.java	100% <100%> (ø)	601 <7> (+2)	⬆️

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[coveralls]

Coverage remained the same at 100.0% when pulling 3cfa609 on terhesb:first into 8c3eedd on javadev:master.

[javadev]

LGTM