Implemented setCharacter() function

+Added RichText::setCharacter(size_t line, size_t pos, sf::Uint32 character)
	Calls the setCharacter() method of m_lines[line]

+Added RichText::setCharacter(size_t pos, sf::Uint32 character)
	Replaces the pos'th character in the line with the character parameter.
Added getCharacter function

+ Added a getCharacter(size_t line, size_t pos) function to RichText
	Returns the pos'th character in the line'th line, as provided by
	  RichText::Line's getCharacter().

+ Added a getCharacter(size_t pos) function to RichText::Line
	Returns the pos'th character in the line.
Fixed get/setCharacter()

* sfe::RichText::Line::getCharacter() now returns properly for characters at the start of a new text.
* sfe::RichText::Line::setCharacter() now works for characters at the beginning of subsequent texts.
* sfe::Richtext::setCharacter() and sfe::RichText::Line::setCharacter() now update the object's geometry.
Prefixed size_t with std:: and replaced the while with a for loop.
Moved get/setCharacter() loop to its own function.

+Added the private function convertLinePosToLocal(std::size_t&) to
    RichText::Line, which gets called from get/setCharacter().

    This function retruns the text that the pos'th character is to be found in
    and modifies the pos value to be the position of the character in the string
    as opposed to in the line as a whole.
Added a getLength function and the assert from #14

+ Added RichText::Line::getLength() which returns the line length as
  an std::size_t

+ Added an assert to RichText::Line::convertLinePosToLocal() to
  protect against out-of-bounds access to the Line (causing UB).
Renamed the index in the getLength function to text.
Added asserts to the get/setChar methods.
Updated the comments on the get/setChar functions 

Given the asserts, attempts to access a character outside of the bounds cause crashes, not UB.
Merge pull request #14 from Skyrpex/feature/random_access

Randomly access characters in sfe::RichText