Refactors the error message of the IsTrue() and IFalse() assertion on Boolean.

Replaces all '\n instead' by '\ninstead' solution-wide (waz too error-prone and time consuming to troubleshoot).
Refactors the error message of the IsBefore() assertion on IComparable.

Makes the IsBefore() method more robust with null operands.
Reformats the error messages of the assertion methods for DateTime.
Reformats the error messages of the assertion methods for Numbers.

Uniformizes the error messages of the IsEqualTo or IsNotEqualTo to replace "Was:... instead of" by "the actual value ... is not equal to ..."
Homogenizes the error messages by starting with "\nThe actual value:\n\t" whenever it is possible.
Reformats the error messages of the assertion methods for Strings.
Homogenizes all the error messages by making them start with "\nThe actual ..."
Gets rid of "i.e.:" within error messages

Introduces some guidelines for writing error messages within the HowToAddANewAssertion.md file.
Updates the Readme.md file with error message samples, and special dedicace to Omer Raviv.
Fixes typo within the Readme.md page.
Fixes typo within the Readme.md page.
Fixes typo within the Readme.md page.
Updates the HowToAddANewAssertion.md file.
Updates the HowToAddANewAssertion.md file.
Merge https://github.com/dupdob/NFluent
Integrates the Lambda and the TimeSpan assertions done by Cyrille. It implied the error messages refactoring + some documentation improvements.

Improves the test coverage of the number part.
Adds Lamba assertion samples within the Readme.md.
Adds Lamba assertion samples within the Readme.md.
Fixes typo within the Readme.md file.
Fixes typo within the Readme.md file.