Tidy up getProperty

[adeutscher]

With PropertyNotFoundException being added as part of #15, getProperty()'s exitOnFail argument doesn't make a whole lot of sense.

Ideas:

• Remove exitOnFail argument to getProperty().
• Change the dictionary of default values that __init__() uses to set default properties to self.defaultProperties instead of self.properties. Preserving our default properties will allow us to use them as a fallback.
• Add a useDefault flag to getProperty() to replace exitOnFail. If true, then we will check self.defaultProperties. If this check fails, raise PropertyNotFoundException.
• Add a defaultOverride flag to getProperty(). If this is given a value, then we will not attempt to look through self.defaultProperties and instead just return this value.

[adeutscher]

Revamped getProperties.

Forgot to document one other detail in ticket: I also corrected an error that I had made with getProperties()' use of PropertyNotFoundException in 7e3a263 while implementing #15. It was being given the originalException argument of an ExceptionWrapper-based exception. Whoops.

[adeutscher]

Reference in 24033ce was a typo. #21 was the correct ticket to be reference. The comment was corrected using git commit --amend and git push --force.