Added more methods to Nodes to match the built-in dict methods

[fun4jimmy]

Added new methods:

• Added Nodes.items()
• Added Nodes.itervalues()
• Added Nodes.values()

Added tests and docstrings for Nodes.keys(), Nodes.iterkeys() and Nodes.iternodes().

[codecov[bot]]

Codecov Report

Merging #628 into master will increase coverage by 0.27%. The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master     #628      +/-   ##
==========================================
+ Coverage    78.3%   78.58%   +0.27%     
==========================================
  Files          34       34              
  Lines        2531     2540       +9     
==========================================
+ Hits         1982     1996      +14     
+ Misses        549      544       -5

Impacted Files	Coverage Δ
jenkinsapi/nodes.py	92.2% <88.23%> (+8.38%)	:arrow_up:

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[lechat]

Many thanks!